51) I forget your name. (= I don't remember your name)
Just as appropriate is: "I've forgotten your name."
2) I forgot your name.
I would use this only with reference to a past forgetting: "I forgot your name last time we met, but I've remembered it this time."
Another one is 3) to be forgetting.......what meanings does it have except I almost forgot?
I don't think it is used in that way. I would use it as a recent, possibly temporary, thing: "I must be getting old - I'm forgetting to do all sorts of things these days."
Student or Learner