If you forget something or forget how to do something, you cannot think of it or think how to do it, although you knew it or knew how to do it in the past.

If you forget something or forget to do it, you fail to think about it or fail to remember to do it, for example because you are thinking about other things.

If you forget something that you had intended to bring with you, you do not bring it because you did not think about it at the right time.

If you forget something or someone, you deliberately put them out of your mind and do not think about them any more.

If you forget yourself, you behave in an uncontrolled or unacceptable way, which is not the way in which you usually behave.

You say 'Forget it' in reply to someone as a way of telling them not to worry or bother about something, or as an emphatic way of saying no to a suggestion.

You say not forgetting a particular thing or person when you want to include them in something that you have already talked about.