1. As I have seen the movie twice, I don't want to see the movie again.
2. As I saw the movie twice, I don't want to see the movie again.
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Which sentence is grammatical?
#1 is correct.
Extra credit: what is bad about #2?
Is it because of the tense? #2 uses the past tense in the subordinate clause.
Both sentences are grammatically correct, but there is a difference in meaning between them.
1. "As I have seen the movie twice, I don't want to see the movie again."
In this sentence, the speaker is using the present perfect tense ("have seen") to talk about a past action or experience that has a connection to the present. The speaker is emphasizing that they have already seen the movie twice, and as a result, they don't want to see it again.
2. "As I saw the movie twice, I don't want to see the movie again."
In this sentence, the speaker is using the simple past tense ("saw") to talk about a past action or experience. The speaker is stating that they saw the movie twice in the past, and as a consequence, they don't want to see it again.
Both sentences convey the same idea, but the first sentence (with the present perfect tense) places more importance on the connection between the past action and the present situation.