This command helped me to get MP4 file from MKV file:
ffmpeg -i "input.mkv" -c:v copy -c:a copy "output.mp4"
This command will help to extract subtitle from MKV file.
ffmpeg -i "input.mkv" -map 0:s:0 output.srt
The above command will extract first subtitle. MKV file contains more than one subtitle track and if you want to extract one of them. Then you can replace “0:s:0” with “0:s:1” to extract second subtitle track, use “0:s:2” for third and so on.