ffmpeg - Subtitles - Add subtitles to MP4 vs MKV

Different syntax is used for MP4 vs 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

NOTE: Notice the -c:s mov_text.

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: Notice the -c:s srt.

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”

NOTE: .srt files must be imported with -c:s copy not with -c:s mov_text