Adjectives and adverbs
Adjectives are modifiers for nouns and adverbs are modifiers for verbs, adjectives, and other adverbs. Not all languages distinguish them, but English does in both grammar and word formation. Grammatically, adjectives precede the noun they modify, whereas adverbs might precede or follow the verb they modify, depending upon the specific adverb (although there are a small number of exceptions). English also has a means of converting adjectives into adverbs: the addition of the suffix <-ly> changes an adjective to an adverb (in addition to moving it to the appropriate place in a sentence).

Occasionally, people use adverbs with verbs that require an adjective.

1. "I feel badly" - the speaker has an impaired sense of touch (likewise: "I hear badly")
2. "I feel bad" - the speaker is ill or upset (likewise: "I feel happy")

The latter is, of course, the meaning most people try to convey. It's unclear whether this example shows the misuse of adjectives/adverbs or the fairly common use of "feel" as a copula verb (whose complement refers to its subject). That is, "feel" is often used with a meaning very close to "be." "I feel sick" is the equivalent of "I am sick" and using "I feel sickly" would be odd, for most native speakers. A better example might be something like: "I drive decent", with a meaning of "I drive decently" or "I drive well."

As well, confusion often occurs between good, well (adj.), and well (adv.).

1. "I feel good" - a good mood
2. "I feel well (adj.)" - good health (though this is often replaced by "I feel good" in everyday speech with little ambiguity in meaning)
3. "I did well (adv.)" - success

There are other ways of changing words from one lexical class to another. Nouns are easily transformed into verbs by moving them to the appropriate position in a sentence, and then conjugating them according to the default paradigm. Nouns can also be changed to other kinds of nouns (<-er>, <-ist>), into adverbs of state/condition (<-ness>), and into adjectives (<-ish>, as in "bullish"). Verbs can be turned into adjectives with <-ing> ("dancing school"), into adverbs with <-ly>, and sometimes even into nouns with <-er> ("dancer", "listener").

These processes provide the English language with greater flexibility in choosing words, expanding vocabulary, and re-shuffling words to add subtlety of meaning that might otherwise not be available in an analytic language.