När lägger man till ly på ett verb i engelskan?

-edly. An apology is perhaps due for 'setting out a stramineous subject' at the length this article must run to; but some writers certainly need advice on it (Women and girls stayed their needles while the Liberal leader's wife and daughter chatted informedly with them), and few have time for the inductive process required, in default of perfect literary instinct, to establish sound rules.
Experiments in unfamiliar adverbs of this type (as embarrassedly, boredly, mystifiedly, biassedly, painedly, awedly) lay the maker open to a double suspicion: he may be NOVELTY-HUNTlNG (conscious, that is, of a dullness that must be artificially relieved) or he may be putting down the abnormal in the belief that it is normal and so betraying that his literary ear is at fault.
The following is offered as a fairly complete list of the standard words; there are some hundreds of others to which there is no objection, but these will suffice to test doubtful forms by. The list is in three parts, first adverbs from adjectives in -ed, secondly adverbs from adjective-noun compounds in -ed, and lastly adverbs from true past participles.
It will probably be admitted by everyone that the list is made up wholly of words known to be in the language already and not having to be manufactured for some special occasion with doubts about their right to exist. Most readers will admit also that, while it is physically possible to say any of those starred without allowing a separate syllable to the -ed-, the only ones actually so pronounced by educated persons are those with two stars; fixedly, for instance, demands its three syllables, and unconcernedly its five.
1. From adjectives in -ed: belatedly, benightedly, conceitedly, crabbedly*, crookedly*, dementedly, dewedly*, doggedly*, jaggedly*, learnedly*, nakedly, raggedly*, ruggedly*, sacredly, stiltedly, wickedly, wretchedly*.
2. From adjective-noun compounds in -ed: -bloodedly (cold-b. etc.), -fashionedly** (old-f. etc.), -handedly (open-h. etc.), -headedly (wrong-h. etc.), -heartedly (warm-h. etc.), -humouredly** (good-h. etc.), -mindedly (absent-m.), -naturedly** (ill-n. etc.), -sidedly (lop-s. etc.), -sightedly (short-s. etc.), -spiritedly (low-s. etc.), -temperedly** (ill-t. etc.), -windedly (long-w. etc.), -wittedly (slow-w. etc.).
3. From true past participles (including some with corresponding negative or positive forms in equally or less common use, which need not be mentioned): abstractedly, admittedly, advisedly*, assuredly*, avowedly*, collectedly, confessedly*, confoundedly, connectedly, constrainedly*, consumedly*, contentedly, cursedly*, decidedly, dejectedly, delightedly, deservedly*, designedly*, devotedly, disappointedly, disinterestedly, disjointedly, dispiritedly, distractedly, excitedly, fixedly*, guardedly, heatedly, hurriedly**, jadedly, markedly*, misguidedly, perplexedly*, pointedly, professedly*, repeatedly, reputedly, resignedly*, restrainedly*, rootedly, statedly, unabatedly, unaffectedly, unconcernedly*, undauntedly, undisguisedly*, undisputedly, undoubtedly, unexpectedly, unfeignedly*, unfoundedly, uninterruptedly, unitedly, unreservedly*, unwontedly.
The upshot is that, among the hundreds of adverbs in -edly that may suggest themselves as convenient novelties, (a) those that must sound the e are unobjectionable, e.g. animatedly, offendedly, unstintedly; (b) of those in which the e can (physically) be either sounded or silent none (with the exception of those in classes (c) and (d) below) are tolerable unless the writer is prepared to have the e sounded; thus the user of composedly, confusedly, dispersedly, pronouncedly, absorbedly, and declaredly, will not resent their being given four syllables each, and they pass the test; but no one wll write experiencedly, accomplishedly, boredly, skilledly, or discouragedly, and consent to the ed's being a distinct syllable; they are therefore ruled out; (c) hurriedly suggests that such forms as palsiedly, worriedly, variedly, frenziedly, and studiedly (from verbs in unaccented -ў) are legitimate; (d) words in unaccented -ure, -our, or -er, seem to form passable adverbs in -edly without the extra syllable, as measuredly, injuredly, perjuredly, labouredly, pamperedly, bewilderedly, chequeredly; most two-starred words in the second part of the standard list answer to this description; (e) few if any from verbs in -ȳ, or from those in -ble, -cle, etc., as triedly, satisfiedly, troubledly, puzzledly, are endurable.
These conclusions may be confirmed by comparing some couples of possible words. Take dementedly and derangedly, open-handedly and open-armedly, admittedly and ownedly, dispiritedly and dismayedly, delightedly and charmedly, disgustedly and displeasedly. The reason why the first of each couple seems natural and the second (except to novelty-hunters) unnatural is that we instinctively shrink from the ed syllable (archaic when phonetics allow the e to be silent) except in established words; charmedly as a disyllable is felt to flout analogy, and as a trisyllable is a bizarre mixture of the archaic and the newfangled.

-ly. 1. For the tendency among writers and speakers who are more conscientious than literary to suppose that all adverbs must ens in -ly, and therefore to use hardly, largely, strongly, etc., where idiom requires hard, large, strong, etc., see UNIDIOMATIC -LY.
2. For participial adverbs like determinedly, see -EDLY.
3. It was said in the article JINGLES that the commonest form of ugly repetition was that of the -ly adverbs. It is indeed extraordinary, when one remembers the feats of avoidance performed by the elegant-variationist, the don't-split-your-infinitivist, and the anti-preposition-at-ender, to find how many people have no ears to hear this most obvious of all outrages on euphony. Not indeed on euphony pure and simple, but on euphony and sense in combination; for as many -ly adverbs as one chooses may be piled on each other if one condition of sense is fulfilled—that all these adverbs have the same relation to the same word or to parallel words. We are utterly, hopelessly, irretrievably, ruined; It is theoretically certain, but practically doubtful; He may probably or possibly be in time. These are all irreproachable. In the first, each of the three adverbs expresses degree about ruined; in the second, each limits the sense of an adjective, the two adjectives being contrasted; in the third, the two give degrees of likelihood about the same thing, that is to say, in all three cases the -ly adverbs are strictly parallel. Euphony has nothing to say against repetition of -ly if there is point in it, which there is if the adverbs are parallel. But, when parallelism is not there to comfort her, Euphony at once cries out in pain, though too often to deaf ears.
Russian industry is at present practically completely crippled. Practically is not marching alongside of completely, but riding on its back; read almost. / He found himself sharply, and apparently completely, checked. Sharply and completely, by all means; but not apparently completely; read as it seemed. / Maeterlinck probably and wisely shrank from comparison with 'Hérodias'. Though probably and wisely both apply directly to the same word shrank, their relation to it is not the same, probably telling us how far the statement is reliable, and wisely how far the course was justified; read It is probable that Maeterlinck wisely shrank.

unidiomatic -ly. As the lapses from idiom here to be illustrated probably owe their origin to the modern wider extension of grammatical knowledge, it may be prudent to start by conciliating the sticklers for grammar and admitting that a -ly is sometimes missing where it is wanted. So: The Carholme course, shaped very similar to the Doncaster Town Moor, is one of the best in England. / Proceedings instituted by the local Education Committee against the mother for neglecting to send her girl to school regular. / I hope that most teachers in the present day have learnt to read the Old Testament (thanks to the higher critics) different from the way l was taught to read it in my youth. / Surely no peace-loving man or woman will deny that it would be advisable to prevent strikes and lock-outs consistent with the principles of liberty as set forth by John Stuart Mill.
But, if grammar is inexorable against consistent and different and the rest, it would in the following sentences allow contrary and irrespective without a frown, while idiom for its part would welcome them: The provision is quite inadequate and very grudgingly granted, and often, contrarily to the spirit of the Act, totally denied. / Loyal obedience is due to the 'powers that be', as such, irrespectively of their historical origin. / His method is to whitewash them all vigorously viith the same brush, irrrespectively [Sic] of differences in the careers and characters of his heroes. Contrary and irrespective are among the adjectives that have, with others mentioned in UNATTACHED PARTICIPLES and in QUASI-ADVERBS, developed adverbial force; to ignore that development is bad literary judgement, but, among the mistakes made with -ly, one of the least.
A degree worse is the use of a -ly adverb where idiom requires not an adverb at all, but a predicative adjective. See LARGE(LY) for the phrases bulk and loom large, and substitute adjectives for adverbs in the four following quotations: In neither direction can we fix our hopes very highly. / This country was brought much more closely to disaster at sea than ever the Allies were on land (much closer). / It is a gigantic labour before which the labours of Westphalia, of Utrecht, of Vienna, pale insignificantly. Sometimes indeed it would not be easy to say whether a predicative adjective or an adverb is more idiomatic. It does not matter which we call hard, bright, and wide in It froze hard, the fire burned bright, the doors were flung open wide; the adverbial or adjectival form can be used with equal propriety in He played the melody loud(ly), it was dangling loose(ly), they sat idl(e)(y) by the fire, when trenching dig deep(ly). Pedantic criticism is often heard of harmless injunctions such as Drive slow, Hold tight, Turn sharp left: there is an old joke about a drowning lady who when adjured by her rescuer to hold tight 'murmured say tightly as she went down for the third time'. If such phrases really need to be justified grammatically, this tendency for the adverb and adjective to merge affords a colourable plea.
Yet a little worse is the officious bringing up to date of such time-honoured phrases as mighty kind, sure enough and safe and sound: Still, it is mightily kind of the Morning Post to be so anxious to shield the Labour Party from the wrath to come. / We begin to remember the story of the detective who died murmuring to himself 'More clues!' and towards the end of the book, surely enough, more clues there are. / I hope your daughter will get home safely and soundly, Mrs. F.
But much more to be deprecated than all the particular departures from idiom already mentioned is the growing notion that every common adjective, if an adverb is to be made of it, must have a -ly clapped on to it to proclaim the fact. Of very many that is not true; see for instance DEAR, DIRECT, MOST, PRETTY, RIGHT, and STRAIGHT. Two such words may here be taken for special treatment, much(ly) as the least, and hard(ly) as the most, important of all. We all know that much can be an adverb, and probably most of us would guess that muchly was a modern facetious formation, perhaps meant to burlesque the ultra-grammatical, and at any rate always used jocosely. We should be wrong; it is over 300 years old. Its earliest use was serious, and even now it may occasionally be met in contexts where the point of the joke is not apparent: Many players who were in the habit of relying muchly upon the advice of their caddies found themselves completely at sea. Nevertheless, as it seems from the OED to have lain dormant for over 200 years, our guess is not so far out, and its revival in the 19th c. illustrates the belief that adverbs must end in -ly. Muchly does not often make its way into print, except in dialogue as a recognized symbol of the mildly jocose talker, and has been worth attention only in contrast with hardly. That, as will appear, is substituted in print for the idiomatic hard neither seldom nor with any burlesque intention, but seemingly in ignorance. Ignorance that hard can be an adverb seems incredible when one thinks of Hit him hard, Work hard, Try hard, and so forth; the ignorance must be of idiom rather than of grammar. Neglect of idiom is, in this case, aggravated by the danger that hardly, written as meaning hard, may be read as meaning scarcely; for some proofs that danger is real, see the article HARDLY. The examples that here follow are free from such ambiguity, but in each of them idiom demands expulsion of the -ly: How hardly put to it the Tories are for argument is shown by . . . / Another sign of how hardly the great families are pressed in these times. / The invasion of Henley by the fashionable world bears very hardly on those who go only for the sport. / They have been as hardly hit as any class in the community by the present state of trade. / If there is a man more hardly hit by existing conditions than the average holder of a season ticket he is hard to find (harder hit).
Other such adverbs are WIDE, late, deuced, HIGH, each spoilt in the appended extracts by an unidiomatic -ly: And then he'd know that betting and insurance were widely apart. / Several drawings in the new volum [Sic] are dated as lately as August and September, 1922. / I bite it—it is deucedly big—I light it and inhale. / M. Millerand has played highly, but he has lost his stake. MIDDLING, soft, FAIR, and SHARP, are specimens of the many others that might be named.

Jag anser att snabb och snabbt i följande två satser (utan annan information tillgänglig än att det handlar om att exemplifiera kongruens) tillhör samma ordklass:

1. Den är snabb.
2. Det är snabbt.