She knew he will leave.

[giddyman]

I know "She knew he would leave," is correct.

If he is still here and didn't leave yet, can I say "She knew he will leave"?

 

[Tarheel]

I know "She knew he would leave," is correct.

If he is still here and HASN'T LEFT yet, can I say "She KNOWS he will leave"?

Yes.