If you want say 'My car has broken down' in Spanish you must say the following sentence:
'Mi coche se ha roto'
The vocabulary is the following:
Mi: (pronoun) My
coche: (noun) car
se: (pronoun)
ha: (verb) to have
roto: (verb) break
Notice that depending in the spanish-speaking country that you are, you should use a different words for 'car':
-Spain: coche
-Latin America: carro or auto