what is the grammatical difference between the following prepositions : at and in
If you look at following totally random examples, you will see how varied the uses can be:
I'm in France, at a party.
I'm at my friend's house and I'm in the kitchen.
I'll do it in 3 hours, at 3 o'clock.
I'm at work, in a really big building!
As you can see, it's not even so simple as saying that you use one or the other for location, for instance. If that was the case, sentence 2 would not be constructed the way it is!