I have a sentence "I study because I'm a student":
Can I replace the だ in だから with です to make it more polite? In the following way:
Please correct me if my original sentence is wrong.
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The most important verb is the one at the end. If you replace だ with です but don't change する to します, you haven't really gained any politeness; it's just unnatural.
The two below are okay, though. The second is the most polite, best for talking to strangers and people who outrank you socially. The first is good for senpai, who are often basically friends who are older than you. (Note that Japanese people never use the word "friend" (友達) to describe someone more than a year or so older than them, in my experience.)
(You don't need 私は more than once.)