In addition to what others have said, if you're trying to get around the city without a car, I remember from my time in grad school here that the metro felt pretty expensive to take to school every day. Then I learned about the DC circulator, which was just a buck or two per ride - that was like less than a third of what I was paying daily on the metro - and just as fast considering the route. They then made the Circulator free during a longish government shutdown & furlough, which was even better.

Which is all to say, definitely look into the DC bus and Circulator systems! They are amazing for a grad student on a budget and the routes are extensive.