1
× I took a taxi in the airport.
√ I took a taxi at the airport.
We use at with a point or location and in for an enclosed space. If we say that we are in the airport, we mean ‘inside’ the airport buildings. Use at unless you want to specify that it’s ‘inside’ the building.
2
× I saw her in the party.
√ I saw her at the party.
We use at for parties, celebrations, and social events: ‘We met them at the dinner last night.’
3
× I went in the office but the manager wasn’t there.
√ I went into the office but the manager wasn’t there.
We use in to talk about the (static) position of something: ‘The manager was in the office’
We use into to talk about the direction of something that is moving: ‘I went into the office.’ ‘He came into my room without knocking.’