forget or forgot?
我那时候知道但我现在忘了

I use " I knew it at that time but forget now ", later my American friend corrects me by saying " forgot " should be used in this case instead of " forget " ( I knew it at that time but forgot now). Then I get confused : forget or forgot?

Help me make clear of it please.

Question

forget or forgot?
我那时候知道但我现在忘了

I use " I knew it at that time but forget now ", later my American friend corrects me by saying " forgot " should be used in this case instead of " forget " ( I knew it at that time but forgot now). Then I get confused : forget or forgot?

Help me make clear of it please.

Answer

I am afraid your American friends are wrong. 
Correct is: "forget". The reasons are obvious.
knew = back then
forget = now

Answer

In this case, you can use forget or forgot. If the context is in the past, then use forgot, if the present, use forget.

Answer

I'm  a little confused by your question .We often use "forgot" in the past tense ,but this sentense indicates that you forget it now ,not forgot it couple of days ago.By the way ,I'm also an English learner,my teacher taught me like this.Hope it helped.

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