I've read that is possible to say I forget or I don't understand (present tense) even if it seems past. For example:
Q: What's the name of that new restaurant on the corner?
A: I'm sorry. I forget.
The same with I don't understand. The person tells me something, but I can't get the meaning, so I say:
I'm, sorry. I don't understand.
Why and when we can use the present tense with these verbs meaning the past?