Question about paying back a subsidized loan?

I'm an incoming college freshman and I'm planning on taking out a federal subsidized loan (3,500). It says that they won't charge interest while I'm in school and 6 months after I graduate so my question is... Can I pay it back while I'm still in school? If I were to pay it back while I'm still a student or within 6 months after I graduate, I'll only have to pay back the original 3,500 [no interest] I'm borrowing right?