“He was walking along the pavement when I saw him.” In this sentence, “when” is a:

Correct answer: Conjunction. “When” joins the two clauses, so it is a conjunction.

Background: “When” can also be an adverb in questions, such as “When did you come?”

• Adverb: An adverb modifies a verb, an adjective, or an adverb.
• Preposition: A preposition comes before a noun or pronoun.
• Pronoun: A pronoun replaces a noun.

Did you know? Here it links “He was walking…” with “I saw him”.

Memory trick: When joins clauses → conjunction.