Direct answer: The word “when” is an adverb and conjunction used to ask about or indicate time, and it can function in several grammatical roles depending on the context.

Core uses

• Asking about time: "When is the meeting?"
• Introducing a question in indirect speech: "She asked when the train arrives."
• Conjunction to connect clauses about time: "Call me when you arrive."
• Relative pronoun in time-related clauses: "The day when we met was sunny."

Quick notes

• When forms part of wh- questions and relative clauses; it does not itself indicate tense, but the surrounding verb tense does.
• It can introduce subordinate clauses that specify time, conditions, or sequence.

If you’d like, share a sentence and I’ll break down the exact grammatical role of when in that context.