Sometimes there is no auxiliary verb already in the statement. For example, when:

... the verb in the statement is present simple or past simple and is positive. Here we use don't, doesn't or didn't:

... the verb in the statement is to be in the present simple or past simple. In this case we use to be to make the question tag:

... the verb in the statement is a modal verb. Here we use the modal verb to make the question tag:

If the main verb or auxiliary verb in the statement is am, the positive question tag is am I? but the negative question tag is usually aren't I?:

EXERCISE
https://www.elt-els.com/2015/04/question-tags-multiple-choice-test-1.html