Hi John,
Often those two words are used interchangeably but there is is very subtle difference between 'car' and 'parce que' in French which I will try to explain.
When there is a clear cause and effect link between the two clauses you will use 'parce que' -
e.g.
Le vase s'est cassé parce que tu l'as jeté par terre = The vase broke because you threw it on the floor
Henri a été licencié parce qu'il avait insulté son patron = Henri was sacked because he had insulted his boss
'Car' is more subtle, it introduces a supposition or an explanation rather than a direct cause and effect . It is often translated as 'for' / 'since' or even 'as' -
Elle doit être malade car je ne l'ai pas vu depuis trois jours = She must be ill as I haven't seen her for three days
Nous sommes trompés d'adresse car nous sommes allés à Rouen et non pas à Caen = We went to the wrong address since we went to Rouen and not Caen
On ne se parle pas en ce moment car nous sommes fâchés = We don't speak to each other at the moment as we have fallen out
Hope this helps!
