Location / Places
He's at the cash machine.
I'm at the gym.
Is she at the theatre yet?
We're all at the hotel.
We use 'at' with places when the person is in the area of the place. Contrast with 'in'. If you are in the gym, you are 'in' the building. If you are 'at' the gym, you in the area of gym. Note: often you can use 'at', 'on' or 'in' but you can never use 'in' or 'on' when you are only 'at'.
The book's at the side.
I'm meeting them at 9 o'clock.
What time does the show start? At 10pm.
She usually has a nap at 2 in the afternoon.
At the sound of the bell, take out your pencils.
Almost exclusively used for the time of day. In the last example it can be used to express a moment in time (at the sound of the bell, at midnight, at that moment).
I like being at the front of the plane.
Young children prefer being at the back of the bus.
I worship at your feet.
At last he found himself a nice girlfriend.