Learn Russian: Нам нравится комната номер пятнадцать.

The verb нравиться actually means "to be pleasing to." The person who likes something is placed in the dative case, so "we" becomes Нам (to us).

Why is the verb нравится singular if "we" are the ones liking the room?

Because the room (комната) is the grammatical subject of the sentence. Since there is only one room doing the pleasing, the verb takes the singular form нравится. If you liked multiple rooms, it would be plural: нравятся.

Why do we use the counting number пятнадцать here instead of the word for "fifteenth"?

When identifying things like rooms, buses, or buildings with the word номер, Russian simply uses regular cardinal numbers. It is an exact match for how we say "number fifteen" in English.

The word for five is пять. Why is there no soft sign in the middle of пятнадцать?

This is a tricky spelling rule! The numbers 11 through 19 are built by adding -надцать to the base number, but for 15 (пятнадцать) and 19 (девятнадцать), the middle soft sign is dropped. They only keep the soft sign at the very end.