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."
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!"