Learn Finnish: Ravintola on hotellin vieressä. - The restaurant is next to the hotel.

This is because of the word vieressä (next to). In Finnish, most location words like this are postpositions. They require the noun they connect to to be in the genitive case (the "possession" case), which usually ends in -n. So, you are essentially saying "in the hotel's adjacent space".

Does vieressä always have to come after hotellin?

Yes! English uses prepositions that go before the noun (like "next to the hotel"). Finnish mostly uses postpositions that go after the noun. So the order must be hotellin vieressä.

I noticed vieressä ends with -ssä, which usually means "in". Is that a coincidence?

Not at all! Many Finnish postpositions are based on nouns and use standard location endings. The -ssä ending means stationary location. If you were moving to the side of the hotel, the postposition would change its ending to show movement: hotellin viereen.

To make the genitive form, do I always just add an -n to words ending in -i like hotelli?