If the morphemic structure is taken as a ground of classification, adverbs fall into six groups:

1) The first largest group is made up of those adverbs formed from derived and base adjectives by adding the suffix -ly: evenly, smoothly, glamorously, beautifully, wonderfully, etc.

2) The second group originally very small, but in present-day English exhibit­ing signs of rapid growth includes those formed by adding the derivational suffix -wise to nouns. A few adverbs of this type are well-established words (clockwise, otherwise, likewise); others are recent coinages or nonce-words like weather-wise and plane-wise.

3) The third group consists of adverbs formed by the addition of the deriva­tional suffix -ward(s) to a limited number of nouns (e.g. homeward(s), toward(s), forward(s), backward(s), etc.). Most adverbs of this group have two forms, one with the final s and one without, variously distributed. The forms without s are homonymous with adjectives: a forward estimate, he movedforward.

4) Next comes a group of adverbs formed by combining the pronouns some, any, every and no with a limited number of nouns or pronominal adverbs, such as someplace, anyway, everywhere, nowhere, etc.

5) Then comes another relatively small group of adverbs that includes words that are formally identical with prepositions: about, around, up, down, be­low, above, over; etc.

6) The last group is the miscellaneous class of adverbs that have no formal signals at all to distinguish them in isolation. They are classed as adverbs because of their position in the sentence. Many adverbs in this group are fairly frequent in occurrence: always, often, now, then, here, there, etc. Some members of this type are homonyms of other parts of speech, such as home, late, fast, loud, early, etc.

Some scholars distinguish between so-called “merged” or “separable” adverbs. The term “merged” implies the fact that such separable compounds are lexically and grammatically indivisible and form a single idea. On the ground of their structure, “separable” compounds may be classified as follows:

1) preposition + noun: at hand, at random, by heart, on foot, in turn;

2) noun + preposition + noun: hand in hand, day by day, face to face, word for word;

3) preposition + substantivized adjective: at last, at first, at large, in full, in vain, of late;

4) preposition + verbal noun: on the move, on the run, in a rush, at a guess;

5) proposition + numeral: at once, at one, by twos;

6) coordinate adverbs: by and by, on and off, on and on, again and again;

7) pronoun + adjective (participle): all right, all told;

8) proposition + pronoun: after all, in all, at all.

One may notice that most adverbs of the abovementioned groups may be reasonably referred to as grammatical idioms. This claim may be corroborated by the unusual absence of the article before their noun components and specialized use of the noun in its singular form only: at hand (but not at the hand, or at hands which may occur in free prepositional word combinations), to date (but not to the date), at first (but not at the first), etc.