1. "Forget to do something" is used when you have intended to do something but have forgotten to do it.
2. "Forget doing something" is used when you have actually done something but later forget that you did it.
3. The base form of the verb "forget" is used for present tense.
4. The past tense of "forget" is "forgot".
5. The past participle form of "forget" is "forgotten".
