The semantic characteristic of adjectives is that they modify nouns, that is, they give us more information about the nouns they occur with. Examples are good, large, wonderful, notorious and long.
Their morphological characteristic is that most of them, but not all, can form comparatives and superlatives. From short, we can make shorter and the shortest. Very generally, this applies to adjectives of one syllable. The comparative and superlative of longer adjectives are formed by placing more and the most in front of them. The comparatives and superlatives of a few adjectives are irregular, such as good, better and the best.
Syntactically, adjectives can occur in front of a noun, as in a warm day, when they are described as attributive. They can also occur after verb, as in the day was warm, when they are known as predicative.
As adjectives modify nouns, so one function of adverbs is to modify verbs. They add detail to whatever it is the verb is describing, in terms of manner (elegantly), place (there), time (yesterday), degree (intensely) and frequency (occasionally).
It is often possible to recognise adverbs morphologically by the ending –ly, as in slowly, necessarily and finally. Other adverbs, such as tomorrow, soon, and well give us no such clue. Syntactically, an adverb can occur at the beginning or end of a clause, or in the middle, depending on the kind of adverb it is.
‘The Cambridge Guide to English Usage’ divides adverbs into the following four groups:
Adjuncts are adverbs that modify verbs, as described above.
Subjuncts modify adjectives and other adverbs in ways that either soften or intensify the words they modify. Among the former are fairly, rather and somewhat, and among the latter are extremely, most and so.
Disjuncts tell us something about what the speaker or writer thinks about the content of an entire sentence. They include adverbs such maybe, possibly, probably and surely that indicate the likelihood of something happening, and those, such as fortunately, mercifully, regrettably and worryingly that express an attitude towards the event described.
Conjuncts are adverbs like also, however, therefore, and thus which express logical relationships between sentences, such as addition, contrast and causation.