The following questions are regarding how a verb have is to be handled in a tag question. GMATCLUB grammar mentions - " The verb have can be used as the main verb (I have a headache), or as an auxiliary verb (Jenny has received her diploma). When have functions as the main verb, a form of do, does, or did must be used (You have a headache, don’t you?)"
Now, let look at some of the questions from Exercise 19:
1. We’ve already taken that test, ______________________?; the answer given is "haven't we" (why is "didn't we" not used here. Is it because of the presence of another verb "taken"?)
2. You have been to Washington D.C., ______________________?; the answer given is "haven't you" (why is "don't you" not used here. Is it because of the presence of another verb "been"?)