Learn Finnish: Me asumme nyt hotellissa. - We live in the hotel now.

To say "in a hotel", you simply take the basic word hotelli and add the "in" ending, which is -ssa. Because hotelli is a regular, non-gradating word, you don't need to make any changes to the core word before adding the ending.

Why does hotellissa use the -ssa ending instead of -ssä?

This is due to vowel harmony. The word hotelli contains the back vowel o, so its ending must also use a back vowel (a), making it -ssa. The vowels e and i are neutral; they don't change the vowel harmony if a strong back vowel like o is already present.

The translation says "in the hotel", but how do we know it's not "in a hotel"?

Finnish does not have articles like "a" or "the". The word hotellissa can perfectly mean either "in a hotel" or "in the hotel". You figure out which one is meant based on the context of the conversation.

Why do we use the verb asumme for "we live" here?

In Finnish, the verb asua means to live in the sense of residing in a physical place, like an apartment, a city, or a hotel. There is another verb, elää, which means to be alive or to experience life. Because we are talking about a place of residence here, asumme is the correct choice.