Word formation: prefixes, suffixes, compounds

Word formation lets us build new words from a base (root) word by adding affixes or by joining words together. A prefix goes before the base and usually changes the meaning, especially the negative prefixes un-, in-, im-, ir-, il-, dis- and mis- (happy → unhappy, possible → impossible, regular → irregular, agree → disagree). A suffix goes after the base and usually changes the part of speech: -tion, -ment, -ness and -ity make nouns; -ful, -less and -able make adjectives; -ly makes adverbs; and -ise/-ise make verbs (decide → decision, happy → happiness, care → careful, quick → quickly, modern → modernise). A compound joins two words into one idea, written as one word (toothbrush), with a hyphen (well-known), or as two words (bus stop). In exams you must choose the right derivative AND spell it correctly, watching for spelling changes such as dropping a final 'e' or changing 'y' to 'i'. For example: from the verb 'decide' we form the noun 'decision', and from the adjective 'happy' we form the noun 'happiness' (y → i).