Why would you use tú when speaking to a classmate?

You would use “tú” when speaking to a classmate because they are the same age as you and you can be informal with them. But it it was a teacher or someone “higher” than you you would use “usted”

You have to fry the plantains.

I'm not sure if you mean for us to translate or to answer. but i'll translate as these are questions for you to answer. 1. how long has it been since you've read a book? . 2. how long has it been since you've eaten breakfast? . 3. how long has it been since you've talked with your spanish professor?