Are interest rates on loans fixed for the life of the loan?
Some are. You can find out how interest is calculated on the website.
What are the requirements to be eligible for a Direct Subsidized Loan?
You must be enrolled at least half-time at a school that participates in the Direct Loan Program. You must also be enrolled in a program that leads to a degree or certificate awarded by the school. Direct Subsidized Loans are available only to undergraduate students who need financial aid.