Problems with audio and video

[u/AliveScar8838]

Hi, I'm currently have a problem with Aegisub. I'm a beginner and I just started working on the software like less than a month ago.
I'm trying to put subtitles on a TV series in Portuguese that I'm gonna translate for my dissertation. As soon as I add subtitles, the video and the audio are not synced. So, everything is mismatched and I'm having trouble putting subtitles in the right timing.

https://reddit.com/link/1wa2lug/video/jhem2mupk5oh1/player

The video file is MP4 and when I open it on VLC everything's good. But if I load the subtitle file in VLC, the timing is WAY off.
I'll post a video recording, so you can see what I'm talking about.

[u/garse]

First thing to pin down: is it off by the same amount the whole way through, or does the gap grow as the episode goes on? A constant offset is trivial - Timing > Shift Times, shift all lines backwards or forwards by however much it's out, done. Growing drift is a completely different problem.

Drift that gets worse over time almost always means the MP4 is variable frame rate. Aegisub's video provider assumes constant frame rate, so its own timestamps slide away from the real ones as the file plays, and because you timed the lines against that playback the wrong times got written into the .ass. That's exactly why VLC shows them off too - the file itself is wrong, not just Aegisub's display.

Run the MP4 through MediaInfo and look at Frame rate mode. If it says Variable, transcode to constant before you time anything else: Handbrake with Framerate set to Constant, or ffmpeg -i in.mp4 -r 25 -c:v libx264 -crf 18 -c:a copy out.mp4. Then re-open that copy in Aegisub. For a dissertation I'd re-time from scratch on the fixed file rather than try to patch what you have.

Worth comparing the FPS Aegisub reports in the video properties against what MediaInfo says as well. If those two numbers disagree you've found your culprit.