Learn Portuguese (Portugal): O carro está na rua.

It is a contraction of the preposition em (in/on/at) and the feminine definite article a (the). Because rua (street) is a feminine noun, we combine em + a to make na.

Can I just say em a rua instead of na rua?

No. In Portuguese, whenever em is followed by a definite article (o, a, os, as), they must contract. You will never see em a written or spoken as two separate words; it must always become na.

Does na mean "in the" or "on the"?

It can mean both! The Portuguese preposition em (and its contractions) covers "in", "on", and "at". Portuguese doesn't distinguish between being "in" a street or "on" a street the way English does, so na rua covers both meanings.

Why use está instead of é to describe where the car is?

In Portuguese, we use the verb estar (which we usually use for temporary states) to talk about the physical location of people, objects, and even permanent places like cities. So, to state where the car is located, we use está.