What's the difference between 'forget buying' and 'forget to buy'?

Use forget + ~ing when you did something in the past but don't remember it.

Since 'forget' is the verb, you can use it in any tense. Present and future tense are most common.

While 'I forget doing ...' is grammatically correct, it is more common to say 'I don't remember doing ...'

I don't remember CALLING you. I did it because you have a voicemail from me, but I just don't remember.

Why don't you remember doing something? Maybe you were tired, drunk, sick, or just too busy.

My car is on a snow pile! How did it get there? I don't remember PARKING there!

That's a strange hat! How did it get in my house? I don't remember BUYING it.

Use forget + ~to when you must do something but do not do it.

Since 'forget' is the verb, you can use it in any tense.


Oops! I forgot TO CALL my mother! Yesterday was her birthday, and she was expecting a call from me!

Don't forget TO STUDY gerunds and infinitives!

Oh no! I forgot TO TURN OFF the faucet before I left my house! I'm going to have a huge water bill this month.

Oops! My phone bill was due yesterday, but I forgot TO PAY it. Now I will have to pay a late fee.