For this sentence, I'm not sure whether to use had or have:
Ever since I saw Harry Potter, I had/have been interested in Muggle Studies.
"Have" sounds better, but "had" and "saw" are both past tense.
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If you were interested but not anymore:
Ever since I saw Harry Potter I had been interested in Muggle Studies. But then I read a little bit in a Muggle Studies textbook, and I'm not interested anymore.
If you're still interested:
Ever since I saw Harry Potter I have been interested in Muggle Studies. I'm hoping to enroll in Muggle Studies when I get to college.
The correct word is "have."
I think the tense confusion comes from the fact that, in the example, you are still currently interested in Muggle Studies, thus you do not use past tense (despite the beginning of your interest being in the past, with the past-tense verb "saw").
"Have" is used in present-tense like you said, but the verb used here is "been" and "have" is the helping verb.
If we think about a past-tense verb, like "have tried" versus "had tried" in the sense of "I have tried to fix the problem" versus "I had tried to fix the problem." The first talks about a problem that I still seem to be trying to fix, an ongoing action still happening in the present. The second is a problem from the past or that I have given up on trying to fix. Using the past tense means that it has ended.
I probably talked too much, but I hope this helped.