As we have seen before, a word always belongs to a certain Word Class. The biggest and most important ones being: nouns, verbs, adjectives & adverbs.
Many words, however, belong to more than one word class, depending on how they are used in a sentence.
For example, ‘book’ can be used as a noun or as a verb:
‘It’s an interesting book.’ (noun) ‘We ought to book a holiday soon.’ (verb)
And ‘fast’ can be used as an adjective or an adverb:
‘He loves fast cars.’ (adjective)
‘Don’t drive so fast!’ (adverb)