Suppose someone says:
Hey, can you help me solve this algebra problem?
What should I say if I want to use the word forget?
- Sorry, I forget how to do algebra.
- Sorry, I've forgotten how to do algebra.
- Sorry, I forgot how to do algebra.
My confusion is between those three. I believe I can use the simple present since what I'm saying is true and it's a fact. However,  and  seem to be true as well at the same time, I can't tell why.