Use Tú when talking in an informal setting (family, friends, one-night stands).
Use Usted when talking in a formal setting or when you need to suck up (doctors, teachers, prison wardens).
To remember which you goes with who, I picture this:
 |
| Tú is wearing a baseball hat because she is informal. Meanwhile, the formal Usted is looking down his nose at you like some kind of Orwellian big brother. |
