This is how to describe a promise. To talk about a promise, you need two pieces of information:

1. who the promise was made to
2. what the promise was.

The "who" part comes right after the word "promise":

She promised him that she'd pay him back.

The information about the promise comes in the clause at the end:

She promised him that she'd pay him back.

Sometimes you can leave out information about who the promise was made to:

She promised that she'd pay him back.

You can also say it this way:

She promised to pay him back.

If the "promise" is being made right now, you use "will":

I promise that I'll take good care of him!

If the promise happened in the past, you use "would":

She promised that she would take good care of him.