Not all videos run the same, due to different frame rates or different cuts. So not every subtitle fits to every video file.
Sometimes, there are hashcodes/releasenames attached to the subtitle. If that is the case, you can find a matching subtitle by simply dragging and dropping your video file in the search field:
Start with this.
If you are not lucky (sometimes there are no hashcodes, or sometimes they are wrong), then you need to go manually:
If the uploader did a good job, they include a release name for which the subtitle is synchronised, for example:
Then, use THIS one to be sure.
Often bluray versions are interchangeable with eachother. Also web-dl's. And also HDTV-versions. But this is no guarantee.
On the other hand, usually those version categories are NOT interchangebale between eachother. For example, almost guaranteed, a HDTV version subtitle will NOT fit on a bluray version.
Here is an extensive explanation. It's in Dutch, but maybe Google can help to translate. Or maybe the images already say enough.
https://forum.opensubtitles.org/viewtop ... 39&t=16548