The word ‘fast’ in the sentence ‘He is a fast writer’ is:

Concept

A word's part of speech is decided by the job it does in a sentence, not by the word alone. A word that modifies (describes or limits) a noun is an adjective; a word that modifies a verb, an adjective, or another adverb is an adverb. The same word can switch roles in different sentences.

Application

1. Find the headword that “fast” sits with: in “He is a fast writer”, “fast” stands immediately before “writer”.

2. Identify what “writer” is: a noun (the person).

3. Ask what “fast” is doing: it tells what kind of writer — it describes the noun “writer”.

4. A word that modifies a noun does the job of an adjective, so “fast” is an adjective here.

Contrast with the other choices

• Adverb: would be right only if “fast” modified a verb, e.g. “He writes fast” (modifies “writes”). Here it modifies a noun, not a verb.

• Noun: names a thing; but “writer” is the thing named, and “fast” only describes it.

• Conjunction: joins words or clauses (and / but); “fast” joins nothing here.

So, by the role it plays — modifying the noun “writer” — “fast” functions as an adjective in this sentence.