I mean, watching a film in French with English subtitles and relaxing is not going to be bad for your French, it will certainly not harm it, but it is a mistake to think you are going to significantly improve your French by doing so.
There is however a way to work with French movies: it's a lot of work, it's tedious and time consuming, but you will learn a lot. To pick movies suitable for working with, consult my list of 10 top easy-to-understand French movies
Here is what you should do:
1 - Pick an appropriate French movie.
a - sound quality and enunciation
Many people think they don't understand the movie because they don't know the vocabulary. Well, it is obviously true if you are a beginner, or if you have selected a movie with a lot of slang and expressions that are not common.
But my experience is that most of the time, the culprit is not so much the vocabulary as the sound quality. Students don't understand well because they cannot hear well; either the actors mumble (for me, Patrick Dempsey, alias "Dr Mac Dreamy" in Grey's Anatomy... I cannot understand 90% of what he says.... It is soooooo frustrating) , or they speak all together, or there is a noisy background, or it's an old movie and the soundtrack is not crisp. So take all this into account when you choose your movie, and select slow-paced stories with dialogues between 2 actors indoors, people whose accent and enunciation you understand. Forget the others!
b - it doesn't have to be a French movie.
As long as it comes with a French soundtrack... and many Hollywood movies do. Actually, picking a movie you already know will help you understand the French better, so it's not a bad idea at all.
c- date and genre
Be careful, also, to train with movies that match the kind of vocabulary you want to learn. You might love Les 400 coups (Truffaut 1959), but be aware that French expressions have evolved since the sixties... In the same idea, a movie like "la Haine" (Kassovitz 1995) taking place in "the hood" may not teach you the vocabulary you need to communicate every day. Go read this blog to understand why you should be cautious with French slang and idioms.
Choose movies with French soundtracks, English subtitles, and if possible French subtitles. That is what's the best. However, don't get frustrated: often the subtitles don't exactly match the dialogue. It happens, but it's a very small percentage of the dialogue.
2 - the method for studying with French movies.
a- You will need a remote control :-)
Put the movie on, without subtitles (if you only have a version with subtitles, take a plastic bag and put it against the TV screen: it will stick and hide the subtitles).
Listen to a sentence or 2, and try to write the dialogue down. If you succeed, great! Move to the next 2 sentences.
If you cannot do it, rewind and repeat at least 4 times. If you still don't get it, write what you hear, phonetically. Then check the French subtitles, or the English ones if that's all you have, and figure out what they said. If you only have English subtitles, you might not be able to do it for every single word, but with the help of a dictionary, you should be able to translate most of the dialogue.
b- Analyze why you didn't understand.
- You didn't know the vocabulary.
But did you get the sound right? If you did, that's already a victory! Can you guess what it means within the scene? If not, go check the English subtitles (or use a dictionary). Repeat the sentence out loud, trying to mimic the actor. Write down the new words of vocabulary in a notebook, in 2 columns: French on one side (don't forget the article if it's a noun), English on the other. Later, test yourself: look at the English word and see if you can come up with the French.
- You couldn't understand the actor.
Then listen to the sentence very carefully, and repeat out loud, trying to mimic the actor exactly. Figure out how you would have said it, and compare it to the way he or she said it. If it's a common word/sentence, write it down in a notebook and try to write how the actor pronounced it. Later, test yourself to see if you can read the word or combination of words correctly.
Many students of mine have used this method, and made great progress. It's a lot of work, but the results are outstanding. If you feel that movies are too hard, start by challenging your understanding with longer audio recordings: start with my French audio books, then I suggest you check out my Poetry Reading and Analysis series, or Fun French Tales, which are both a reachable way to challenge your listening skills.
And take it all with a grain of salt. Chances are that a lot of actors will elude you. I have many "Mac Dreamy's", and it's a pain, but I have to stop the movie and ask Olivier "what the heck did he say?". And Olivier, who's been in an English speaking environement since we was 12 repeats for me. The good thing is that when we watch a movie in French, sometimes the same thing happens to him (I understand and he doesn't), although he is perfectly bilingual. And sometimes, we both have no idea... And then we put on the subtitles!! So, what I mean here is that you are not alone, and it's essential to pick the right audio tool to WORK with... there are many good movies that should be enjoyed for the story and the filmmaking, with the subtitles on - and which are not best suited for language learning purposes.
There is work, and there is leisure. You should have both in your life.