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ffmpeg - Subtitles - Add subtitles to MP4 vs MKV

Different syntax is used for MP4 and MKV.

In both cases you must specify video and audio codec, or just copy stream if you just want to add subtitle.

MP4

ffmpeg -i input.mp4 -f srt -i input.srt \
-map 0:0 -map 0:1 -map 1:0 -c:v copy -c:a copy \
-c:s mov_text output.mp4

ffmpeg -i input.mp4 -f srt -i input.srt \
-map 0:0 -map 0:1 -map 1:0 -c:v copy -c:a copy \
-c:s srt output.mkv

NOTE: If mapping is not used, then adding additional subtitles would overwrite the existing ones.

To add language metadata (insert before output file) “-metadata:s:s:0 language=eng”