BA’s subway system, called Subte, can get you most places you want to go. Just make sure you’re always checking to see where you are and that you’re sitting down when you do. Announcements of stops and stations occur only sporadically and if you’re standing up, you won’t be able to see what station you’ve pulled into, either because your sight-lines are bad, or because there’s a kiosk selling magazines blocking the signs.
My advice is, before you even enter a subway car, to count the number of stops between where you are and where you want to go. Then keep track as you pull into stations. Otherwise, you’re liable to over- or undershoot. Don’t rely on finding maps inside the cars either. Sometimes they’re there, and sometimes it’s just another advertisement for Pepsi. None of this phases the porteños, or the locals, who obviously have the whole route memorized; so don’t be afraid to ask, “Donde estamos, por favor?” if you don’t where you’re at.