Watching films is a fun way to improve your English! First of all, it is a lot more fun that studying with a textbook. There are various resources to help you learn English (or any other language) through films.
While sitting down with a film, we get to listen to real native English. Unlike textbooks (which are excellent for learning grammar and vocabulary), listening to actors speaking the language in a natural way will improve fluency and pronunciation. New words are more likely to be remembered because they are used in a context.
Language and culture are closely connected, and while watching films we can learn about the culture behind the language, too. And we also get familiar with different accents.
One major advantage is that we can choose an interesting film to watch, based on personal preferences (genre, actors, date of release). In case the vocabulary is too complex for you to understand the spoken language, you can always add English subtitles, which is helpful for a better understanding and improving the reading skills, too.
While watching films, we come across informal expressions used in everyday English. When we hear a line that we really like, we can write it down and repeat it. When we say it aloud, we are more likely to remember it.
Beginners can watch videos on YouTube, where you can turn the English subs on whenever necessary. It is recommended to avoid subtitles in your own language.
Some entertaining films can be, at the same time, easy to understand for most learners, when it comes to vocabulary and accent. You can choose for instance, on a lovely winter evening this January, Toy Story or the romantic comedy Love Actually (a rom-com, with a bit of Irish accent as well, used by one of the many characters).
In case films seem too long and uncomfortable to follow, there are many short films in English on YouTube (search for “short films in English”). There are good English short films on shortotheweek.com, too.