3

Since when do you know each other?

vs

Since when have you known each other?

I think people use the first option more often.
however since is a signal word for present perfect.

  • 2
    As an English speaker these sentences could be used but are really awkward. I'd typically ask one of "When/Where/How did you meet?" – MaxW Feb 11 '16 at 7:10
  • when comes with the past simple, hence it indicates the specific time in the past when the action happened. The present perfect doesn't state when action happened. In this case since is a point of time that brings up the completed action up to the present. – Alejandro Feb 12 '16 at 19:51
2

Since when do you know each other?

connotes doubt or even incredulity that the two people in question know each other at all. "Since when" is synonymous with "How long has this been true without my knowing it?" The asker of the question is less interested in finding out the length of time than in communicating their surprise.

Since when have you known each other?

asks when the two people in questions first met. It would normally (but not necessarily) be answered with a specific moment, e.g. "Since 2013" or "Since yesterday"

Another alternative is:

How long have you known each other?

This would normally be answered with a duration of time, e.g. "For three years", or "For a couple of days".

  • +1. Or surprise. Since when do you two know each other? But also putting someone in their place: "Since when do you tell me how to run my life". – Tᴚoɯɐuo Feb 12 '16 at 20:02
0

To ask how long someone has been know the simple question is

How long have you known each other?

In your examples

Since when do you know each other?
Since when have you known each other?

The second alternative would be preferable since it shows knowing in the past continuing to the present, present perfect

Upon meeting two people, the question

Do you know each other?

is appropriate to ask if they are new acquaintances or not

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