Learn Turkish: O bir araba.

In Turkish, there is no separate word for "is" in basic sentences like this. For the third person (he, she, it, or that), the verb is simply understood. Just putting the subject o and the object bir araba together automatically means "That is a car."

Could O bir araba also mean "It is a car"?

Yes! The word o is very versatile. It is a demonstrative pronoun meaning "that" (for things far away), but it is also the standard personal pronoun for "he", "she", and "it". The exact English translation simply depends on the context of your conversation.

Why is the word bir used here, and does it always come before the noun?

The word bir literally means the number "one", but it is also used as the indefinite article meaning "a" or "an". Just like in English, it comes right before the noun it describes to make bir araba ("a car").

Why use o in this sentence instead of bu or şu?

It depends on distance! You use bu ("this") for a car right next to you, şu ("that") for a car a short distance away that you want to draw attention to, and o ("that") for a car that is further away from both you and the listener.