我肯定做sth或者我保证做sth. 是 用must 还是别的情态动词

I must have done sth.表示我肯定做过谋事.
I promise that I have done sth.表示我保证做过某事。

I will definitely do something.
I promise to do something.

must是必须做什么，所以不合您的意思。

i am going to do sth.

I will do something.
I promise to do something.