You can use subdownloader or to install in shell sudo apt-get install subdownloader which is very good to download subtitles for any type of movie/series.

After downloading/installing it the program should be in the Sound & Video section in the applications panel. If you are using Unity by default just press SUPER and type subdownloader and press enter.

When you open the program it will automatically log in to opensubtitles and just search for the movie you want to find the subtitles for or the folder where the video or videos that you want to search the subtitles for. Then select the language for them and download. It will automatically put it in the folder where the video is, renaming it to the video name and enjoy. Open the video with VLC and you should be able to watch the movie with the subtitles.