Correct spelling, explanation: both check in and check-in are correct, but their usage depends on the context and part of speech. When used as a verb, check in is typically written as two separate words. It means to register, report one’s arrival, or confirm attendance at a particular place or event. When used as a noun or an adjective, check-in is written with a hyphen. It refers to the process or location where one registers or confirms attendance.

Definition of check in:
verb, arrive and register at a hotel or airport
Please check in at the reception desk when you arrive.
I need to check in at the hotel before 3 PM to secure my reservation.

Collocations with check in:
Some most commonly used collocations include:
1. Check in online: this collocation refers to the process of registering or confirming attendance for a flight or an event through a website or mobile app before arriving at the location.
Make your travel experience smoother by checking in online before your flight.
2. Check in at the hotel: this collocation is used when arriving at a hotel and going through the registration process to obtain a room key or complete necessary paperwork.
After a long journey, we were eager to check in at the hotel and relax.
3. Check in with someone: this collocation means to make contact or report one’s presence to someone, often to provide an update on a situation or to show concern for their well-being.
I’ll check in with my colleague to see how the project is progressing.
4. Check in on someone: similar to the previous collocation, this one also means to make contact with someone, but with a specific focus on ensuring their well-being or offering support.
Don’t forget to check in on your elderly neighbor during the hot weather.

Correct spelling, explanation: check-in is a noun that refers to the process of registering or confirming one’s arrival at a particular location or event. It is commonly used in various contexts, including travel, hospitality, and events. In the context of travel, check-in typically refers to the procedure passengers go through at airports or hotels to confirm their presence, provide necessary identification or documentation, and obtain boarding passes or room keys. On the other hand, check in is used as a verb, and it consists of two separate words.