Learn Polish: Jesteśmy teraz w kinie. - We are at the cinema now.

Because it follows the preposition w (in/at) to express a location, which requires the noun to take the Locative case. For neuter nouns ending in -o like kino, the Locative ending is -e.

Why does the spelling change to -nie at the end of kinie instead of just -ne?

When adding the -e ending for the Locative case, the consonant right before it often has to "soften". Here, the n in kino softens into ni. That is why the word becomes kinie (pronounced roughly like "kee-nye") rather than "kine".

The translation says "at the cinema". Why use w instead of na?

While English uses "at" for many locations, Polish distinguishes between being inside an enclosed structure and being in an open space. Since a cinema is a building with walls and a roof that you go inside, Polish uses w (in).

Where is the word for "we" in this sentence?

It is built right into the verb! Jesteśmy specifically means "we are". In Polish, you usually drop subject pronouns like my (we) because the verb's ending (-śmy) already tells the listener exactly who is doing the action.