You are going to give your friend some advice. Use should. Your friend always gets a bad score in English tests because he doesn’t like to read. Advise him or her To Read a lot!

Jawaban:

you should study harder to get better grades, that's also for your future.

You should read more books, not only will it broadens your knowledge but it also will improve your vocabulary. Naturally, you would be able to get a better score on the next test.