Subtitles out of sync? Here's how to tell which fix you need
Subtitles out of sync? The fix depends on how they're out, and you can tell in about a minute. Pick two lines, one in the first few minutes and one near the end, and note how far off each one is and whether it's early or late.
Same gap at the start and the end
That's a constant delay, the easy one. Use Shift subtitle timing and move everything by the gap you measured. Subtitles that show up before the line is spoken need a positive number. Ones that show up after it need a negative one.
Fine at the start, worse and worse later
That's drift, and a frame rate mismatch causes it almost every time. Subtitles made for a 25 fps release and played over 23.976 fps video slip by about 4% of the running time, which comes to roughly two and a half minutes for every hour of video. Use the subtitle frame rate converter. Shifting can't fix drift, because the gap keeps growing.
Off at both ends, by different amounts
If the error grows steadily, two-point sync solves it. Give it one early line and one late line, each with the time it should have, and it corrects the gap and the drift in one pass.
Right for a while, then suddenly out
The subtitles were timed to a different edit of the video, usually one with an extra scene or an ad break. Fix each stretch on its own: the shift tool can move separate parts of the file by different amounts.
Why fix the file and not the player
VLC, Plex and most players have a subtitle delay setting. It lasts for one session in one app. Close the video or switch from the laptop to the TV and the old timing is back. A corrected file plays in sync everywhere, including on devices with no delay setting at all.
When no timing fix will work
If the subtitles have lines for scenes your copy doesn't have, or skip scenes it does, they were made for a different cut: an extended edition, a TV version, another release. Timing can't fix that. Find a subtitle file made for your release.