For example, one effect of the English derivational suffix -ly is to change an adjective into an adverb (slow → slowly).
A prefix (write → re-write; lord → over-lord) rarely changes the lexical category in English.
The prefix un- applies to adjectives (healthy → unhealthy) and some verbs (do → undo) but rarely to nouns.
Generally speaking, inflection applies in more or less regular patterns to all members of a part of speech (for example, nearly every English verb adds -s for the third person singular present tense), while derivation follows less consistent patterns (for example, the nominalizing suffix -ity can be used with the adjectives modern and dense, but not with open or strong).
A non-exhaustive list of inflectional morphemes in English: -er, -est, -ing, -en, -ed, -s. Derivation can be contrasted with other types of word formation such as compounding.
For example, the negating prefix un- is more productive in English than the alternative in-; both of them occur in established words (such as unusual and inaccessible), but faced with a new word which does not have an established negation, a native speaker is more likely to create a novel form with un- than with in-.
For example, if comparing two words Thatcherite and Thatcherist, the analysis shows that both suffixes -ite and -ist are productive and can be added to proper names, moreover, both derived adjectives are established and have the same meaning.