Learn Spanish (Latin America): El hombre abre el carro con la llave.

Abre comes from the regular verb abrir (to open). Since we are talking about the man (he, or él), we drop the -ir ending and add -e. If you were saying I open, it would be abro, and you open would be abres.

I thought the word for car was coche or auto. Why use carro?

They all mean car, but they are preferred in different regions. Carro is very common in many Latin American countries, like Mexico and Colombia. Auto is widely used in South America, while coche is the main word used in Spain.

How is the double L in llave pronounced?

In most of Latin America, the ll makes a y sound, like in the English word yes. In some regions, it can also sound a bit like the English j in jump. For standard Latin American Spanish, pronouncing it like yah-veh is perfect.

In English we usually say the man opens HIS car. Can I say su carro instead of el carro?

You can say su carro, but Spanish often just uses the definite article (el or la) when it is already obvious whose object it is. Saying el carro is very natural here because we assume he is opening his own car.