To check in is to show your ticket at an airport so that the airline knows you have arrived, and they can put your bags on the aircraft.
a) We have to check in at 8 o’clock.
b) Please check in at least one hour before your flight leaves.
The check-in (noun) is the place at the airport where you show your ticket and let the airline know that you have arrived.
To check in (or check into something) is to arrive at a hotel reception desk and tell the hotel staff who you are, and collect your room key.
a) We need to check into our hotel before 10pm.
b) I’ll meet you in the hotel restaurant in 10 minutes. I’ll just check in and put my suitcase in my room.
c) Where’s dad?
He’s checking in and collecting our room keys.
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