Since or For? The Timeline Trick

Lovely! You're chatting with someone new at a party. They ask, how long have you lived here? And you say, since three years. They nod politely... but they look a bit confused. Here's the trick. Picture a timeline. Since points to one moment on that line. For measures the whole stretch. Since Monday. Since last summer. Since I was ten. Each one is a single point in time. But for three years. For two weeks. For ages. See? Each one is a length, like a piece of string! Oh, and here's a bonus! Both usually go with the present perfect. I've lived here for three years. I've lived here since last summer. So next time, ask yourself one thing. Can I point at it on a calendar? Yes? Since. No? It's a length, so for. Now, how long have you been learning English?