I've just took an English exam and there was a question I must answer using "since".
How long have you known her?
My answer was "I have known her since February". But then I thought what if I wrote like "I know her since..". The question is that if I can use "since" with "simple present tense".
The second thing made me confused is
"have known" is present perfect and I think the sentence means you don't know her anymore and how long it took to know her. Is that true? I mean, is that different from "how long do you know her?"? If so, can we say "how long did you know her" instead of saying that sentence with "have you known"?
Best Answer
1) Your test answer is correct. It matches tense with the question.
2) "I know her since. . ." is not grammatical. The simple present is not used to encompass a period of past time.
3) Present perfect "have known" is exactly the right tense to express something continuing from the past up to the present moment. "Had [verb]" (past perfect) is used for past {event/situation} which {completed/is no longer the case}.
BUT "know" is a special case, semantically unlike other verbs (e.g. support, date, sleep with—see #5.)
4) "How long do you know her?" is not grammatical (see #2.) So it makes no sense to ask whether this means something different.
5) Yes, one might ask "How long did you know her?", but it does NOT mean you don't know her now, BECAUSE once you "know" her, you cannot later not know her. (You could have a falling-out; you might not see her for 20 years, but you still know her.)
So, "How long had you known her" would be asked in reference to a particular moment in the past:
And "How long did you know her" would be asked in reference to a certain period in the past:
... but both of the above assume that the "knowing" continues now, and into the future.