I have a long-standing doubt on 'when'.

1) Is it a conjunction or preposition?

I don't know whether it is preposition or not, for every verb after it will take -ing or becomes a gerund:

1) Before/After/When studying, don't talk to her.

If it is not a preposition, why does it behave so?

  • Because the simple rule, it can't have more than one verb in a sentence.
    – Zhang
    Commented Jul 12, 2019 at 9:13
  • 1
    @Zhang There is no rule that limits a sentence using "when" to a single verb. For example: "When you drive, you fly". (This could have several meanings, but the most obvious is "when you drive an automobile, you go too fast." That also has two verbs after 'when' come to think of it.) Commented Jul 12, 2019 at 13:17

1 Answer 1


At least according to Merriam-Webster, when can function in four different ways—and none of them is as a preposition.

Also, it doesn't require verbs following it to take an -ing form. The following version of the example sentence would also be fine:

When you study, don't talk to her.

From Merriam-Webster:

1 :at what time
// when will you return

1 a :at or during the time that: WHILE
// went fishing when he was a boy
1 b : just at the moment that
// stop writing when the bell rings
1 c : at any or every time that
// when he listens to music, he falls asleep

: what or which time
// life-long homes for those … who have lived here since when
— Kim Waller

: the time in which something is done or comes
// about troubled his head very little about the hows and whens of life
— Laurence Sterne

In the case of the example sentence, it's being used as a conjunction. Although the first example sentence of Merriam-Webster's definition of its adverbial sense starts with when, it's not being used to join two clauses together.

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