About tense
---I hope I can see you at my birthday party.
---Oh, dear, I ---------------------. When is it?
A. almost forgot B. have almost forgotten

The answer is B.

The person forgot the party before they talked. So "forget" happened in the past. Why not choose A?

Question

About tense
---I hope I can see you at my birthday party.
---Oh, dear, I ---------------------.  When is it?
A. almost forgot     B. have almost forgotten

The answer is B.

The person forgot the party before they talked. So "forget" happened in the past. Why not choose A?

Answer

Why not "A" indeed. No one would say "I have almost forgotten". That's ludicrous.

Answer

I don't think "I have almost forgotten" is grammatically incorrect, but it is not a natural way to speak. Everyone who speaks English would say "I almost forgot!"

I hope this is a more helpful answer. ;-)