Depends on what kind of subs that you are trying to add. And how you want them in the video. vob subs that are overlayed onto the video, srt subs that are a seperate stream in the video.
If you are trying to add a srt file as a substream to a video.
2 examples:
Code:
ffmpeg -i infile.mp4 -i infile.srt -c:a copy -c:v copy -c:s mov_text outfile.mp4
Or
Code:
ffmpeg -i infile.mp4 -i infile.srt -map 0:v -map 0:a -c copy -map 1 -c:s:0 mov_text output.mp4
Need more info.