Let’s say you want to improve your understanding of spoken English. Watching movies and TV Series is a great way to do it, but enabling subtitles (in any language) is an insidious habit.
What you think happens: “I read and listen at the same time.”
What really happens: You are only reading the subtitles, while ignoring the sound.
Yes, you might hear the sound, but you are not listening to it. Your attention is spread too thinly. If you’re learning English, processing English text consumes a hefty chunk of your attention. There’s little left for listening.
You might say, “well, I will only listen. I will only look at them when I need to.” Unfortunately, this doesn’t work either. First, it’s hard to avoid reading text. You need to actively suppress the urge to read, and if you lose attention for a second, you’ll find yourself reading again. But let’s suppose that you manage to ignore the subtitles, and you’re listening again. In general, when we’re listening to speech, understanding can come after a delay of one or two seconds. Until that time, you can’t be sure whether you understood or not. When two seconds pass and you notice that you didn’t understand what has been just said, the subtitle you need is already gone. (You get frustrated and go back to reading.)
What do I suggest instead? Try as follows: disable subtitles, focus hard (yes, it’s supposed to be an effort!), and see how much you understand. If you miss a sentence or two every now and then, don’t worry. If you have a feeling that you missed something important, rewind and try listening to it a few more times. If you’ve listened to it 10 times and you still don’t understand it, check the subtitles, but disable them again afterwards.
If you miss so much that you lose track of the story and/or start zoning out, that’s a signal that what you’re watching is too challenging. Try looking for something simpler, and perhaps shorter. You need to find materials that require a bit of effort, not too little, not too much ‒ it’s up to you to gauge it. Make it progression, once you’ve worked out a simpler one, continue to a more challenging one. If you feel frustrated, go back to an easier one. Bounce back and forth as much as you like.
For example, I find 2-3 minute long comedy sketches excellent for my Portuguese learning. They’re funny, I can analyze them in full, and they do present a challenge. Sometimes it’s really good to focus on a short piece of content, and work out every little bit of it. Then I go back watching something longer, like Disney movies.
P.S. If you’re using subtitles in your native language, it’s even worse, because your brain is confused as to which language it’s supposed to operate in, and most likely locks on the non-English language, shutting English out from your attention.