Learn Croatian: Čekamo lift u prizemlju. - We are waiting for the elevator on the ground floor.

In English, we say "wait for", but in Croatian, the verb čekati (to wait) takes a direct object in the accusative case. You simply "wait the elevator". Since lift is an inanimate masculine noun, its accusative form is exactly the same as its dictionary form.

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

Croatian usually drops subject pronouns like mi (we) because the verb ending already shows who is acting. The -mo ending in čekamo means "we are waiting". You only need to add mi if you want to add emphasis.

Why do we use u instead of a word for "on" like na?

Prepositions often do not translate word-for-word between languages. In English you wait "on" the ground floor, but in Croatian you are "in" it (u).

The dictionary word for ground floor is prizemlje. Why does it change to prizemlju here?

The preposition u here tells us where something is statically located, which requires the locative case. For neuter nouns ending in -e like prizemlje, the locative case ending is -u, making it prizemlju.