Best answer by Maryam
When you download a video, the default name is usually index. The default name for a subtitle file may be subtitles-en, for example. Perhaps that's why your player cannot distinguish your file. This is what I do usually:
- I rename them so that both of them have the same name. For example, they could be lesson1.mp4 and lesson1.vtt.
- In addition, I have installed the recommended player which is VLC Media Player.
- Finally, make sure you've turned on the subtitle option.