Usually, "I've forgotten" is best, but "I forgot" is OK. "forget" is the opposite of "learn", not "know". When something changes from not being in your brain to being in your brain, you learn it. When it is in your brain, you know it. When it leaves your brain, you forget it. After it has left your brain, you have forgotten it. Since we almost never notice the moment when something leaves our brain, we almost never say "forget." Instead, we look at our brains, and see that we don't know something that we used to know. We know that we have forgotten it, but we don't know when we forgot it. "Oh no! I've forgotten his address!" "I used to know a lot of physics, but I've forgotten most of it." "Do you know the Chinese word for ''horse'?"--"No, I've forgotten." To mean "yes, I understand" say "got it". "The meeting is at 9:00." "Got it." To say "I'm not confused anymore", say "I get it." "Multiplication is repeated addition." "What? I don't understand!" "2 times three is two plus two plus two (2*3=2+2+2)" "Oh! Now I get it!"

I forgot or I have forgotten is the past tense. You would use this to say you have forgotten to do something or bring something or it has gone from your memory. "I forgot/ have forgotten my keys" "I forgot /have forgotten yesterdays lesson" "I forgot/have forgotten to tell Mary we won't be home this evening" I forget is present tense often used interchangeably, but it means you do not remember or recall. "I always forget to bring my keys" "I forget" when used by itself as an answer or interjection= you forgot or you do not recall at that time. "I am such a forgetful person" correct: forgotful = Wrong.

I agree with Fatima. And the same counts for "I get it" = present or future "I got it" = past

same goes for get and got for example: 'I get my salary every month.' 'I got my phone yesterday.' when you are referring to understanding something you'll use 'I got it.'

'I forget things very often.' When you are referring to present, or future. 'I forgot my phone at home.' when you are referring to something that has already happened.