forget / forget about sth.

Hi,

could someone tell me what the exact rules for using forget / forget about sth are?

My observation is that forget is used with the preposition only when the verb has the meaning of "stop remembering", and not always at that.

1. I completely forgot about her birthday. (Is about optional here?)

2. I forgot my passport.

3. You are always forgetting my birthday! (In my opinion, this sounds better without the "about" but I may be wrong)

In 1. and 3., forget means "be unable to remember / dismiss for mind". In 2, it means "to leave behind".

Thanks in advance, as always.