Hey Allison, good questions!
1) An unsubsidized Stafford loan will begin to accrue interest as soon it is fully disbursed. That means it will accumulate interest while you’re still enrolled in school. A subsidized Stafford loan on the other hand, will not accrue interest until your school deferment and grace period are over. So it will not accrue interest until you begin payments on it.
2) Subsidized and unsubsidized loans are lumped together for ease of payments but they are still tracked separately. That means that if you do want to make a payment towards one and not the other, you should be able to make arrangements to do so through your servicer. Make sure to contact them and find out the best way to handle that as each Federal loan servicer is going to be a little bit different. It is usually not as simple as just writing a note on your check. You can pay loans online whether you’re a student or a graduate. Now most Federal servicers do have functionality on their websites to allow you to make payments, whether those are your regular monthly payments or just interest payments while you are in school. While most current Federal student loan servicers do allow online payments, the Department of Education has been adding several new servicers lately. So you may want to check with your servicer to make sure that they do allow that. A list of all the Federal student loan servicers can be found at studentaid.ed.gov/repay-loans/understand/servicers. I hope that helps and good luck!