in or at
When do I use in or at?
Elizabeth's answer is about time. We also have to choose between in and at when we talk about locations. I think that most English speakers would have a hard time explaining why we use one or the other. The best strategy for learning their use would probably be to listen to and read so much English that usage patterns would come to sound "natural" and "right".
I am in the U.S.
I am in Philadelphia.
I am in school. (I am attending school.)
I am at school. (I am located at school now and probably engaged in school activities.)
I am at the school. (I am located in or just outside the school building; no reference to what I am doing there.)
I am in the school. (I am inside the school building.)
I am at home. (My current location is home; I might be in or outside the house.)
I am in the house. (I am inside the house.)
I am sitting at the table.
In- usually implies that I am inside or surrounded by my location.
At- usually means I am in or near or next to my location
When I say I am at work, or at school, or at church, it often implies that I am attending the activity that usually takes place at those locations.
For 'in', use it for:
- months / seasons
- time of day
- after a certain period of time (when?)
My birthday is in August.
I always have tea in the morning.
For 'at', use it for:
- a certain point of time (when?)
What time do you go to bed at night?
Let's meet at 3:15.