(Studying morphology) What exactly is the role of inflection?

Certain lexical word classes can take an inflectional suffix to signal meanings and roles that are important to their word class, such as 'plural' in the case of nouns, and 'past tense' in the case of verbs. The following examples demonstrate which word classes can have inflection and what this denotes. The inflectional suffix is highlighted in bold, preceded by the role in brackets. 

Nouns: boy -> boys (plural), boy's boys' (genitive) 

Verbs: live -> lives (singular present tense), lived (past tense), lived (past participle), living (ing- participle) 

Adjectives: dark -> darker (comparative) darkest (superlative) 

Adverbs: soon -> sooner (comparative) soonest (superlative) 

Other classes of words are invariable - they only have one form (the base form/stem). E.g prepositions (of, in, with), conjunctions (if, while, unless) and determiners (the, each, several).