Install FFmpeg
Open a terminal
Run `ffmpeg -i input.mkv -c copy output.mp4`
If the video or audio codec is not compatible with MP4, re-encode with `ffmpeg -i input.mkv -c:v libx264 -c:a aac output.mp4`
If subtitles are needed, use `ffmpeg -i input.mkv -c:v copy -c:a copy -c:s mov_text output.mp4`
Check the output file
Use `ffprobe output.mp4` to verify the container and codecs
If batch converting multiple files, use a shell loop with FFmpeg
If preferred, use HandBrake as a GUI alternative