For the first question :
You can't always use in/at as alternatives. For instance *in home
is incorrect. However, in general, when they are alternatives, at
is more general - I know approximately where the person is, but not exactly. So if eg someone had gone to meet a friend off a train :
- Where's Chris.
- She's at the station.
Here, the speaker doesn't know if she really is in the station (ie inside). He might be - eg in the waiting room of the coffee bar. But he might also be outside - eg waiting on the platform.
Compare too : - Where's Chris?
- She's at the office / She's in the office.
If the speaker were her mother, she wouldn't know exactly where Chris was in the office where she works at the moment of speaking. She might be actually inside her office, in the corridor, in the canteen, in reception - anywhere. But now imagine that the speaker is a colleague, who saw Chris go into the office five minutes ago. He knows she really is inside a specific room - and therefore would say : She's in the office.
is used to mean "inside a specific room or building" while at
just means generally in the location. That's why we can't say *in the home
(except in the sense of a nursing Home). Home is not a specific room/building. It's a concept meaning "the place where you live". If I say that someone is at home
, they could equally easily be inside the house/flat, on the balcony, in the garden, on the stairs, outside throwing away the rubbish, in the garden etc etc. If on the other hand I say that they are "in the house", then I'm am specifying a definite position inside a building.
But notice that at home
doesn't use the article, whereas the other expressions >I've been choosing as examples do. Without the article, things change a bit. In/at
+ place usually emphasises what you're doing in the place rather than the actual location. There's another thread related to this that looks at the meanings expressed by eg He's in prison/hospital
vs He's at the hospital
. See : "go to work" vs "go to THE work"
. It also deals with in/at school
When there isn't an article, though, it sometimes seems random as to whether in or at should be used. So eg : prison/hospital/bed
- only in
is possible : He's in prison/hospital/bed
- only at
is possible : She's at work school
- both in
are possible : We learnt it in/at school