“When” is not a preposition; it is usually a subordinating conjunction and can also function as a relative adverb or an interrogative adverb.

What “when” usually is

  1. Subordinating conjunction (very common)

    • Introduces a time clause.
    • Examples:
      • “Call me when you arrive.”
      • “When the movie ended, we left.”
        In both, “when” links a dependent time clause (“when you arrive”) to a main clause, which is the job of a conjunction, not a preposition.

  2. Relative adverb

    • Refers back to a time noun and introduces a clause.
    • Example: “I remember the day when we first met.”
      Here, “when” modifies the verb “met” and relates back to “day”; it behaves like a relative adverb.

  3. Interrogative adverb

    • Used in questions about time.
    • Examples:
      • “When will you go?”
      • “Since when have you lived here?”

Why “when” is not a preposition

Prepositions must have an object (usually a noun or pronoun) and form a prepositional phrase:

  • “at noon,” “before dinner,” “after the meeting.”

By contrast, “when” does not take a noun/pronoun object; it introduces a full clause with a subject and verb:

  • Not: “when noon” (prepositional pattern)
  • But: “when it was noon” (conjunction pattern).

So although prepositions can indicate time (at, in, on, before, after, during), “when” itself is not classified as a preposition in standard English grammar.