DIRECT STUDENT LOAN

Is it true that once you start paying your fafsa direct loan while you are at school, you have to keep making payments? Or can you pay certain amount of money when you get some? Lets say I have 5000 loan dept and i am a third year student. By chance I managed to make 3000. Can I go head and pay the 3000 to my dept? Or if i start paying back my loan, i have to keep it up untill i am done.