MKVToolnix is a set of tools for editing Matroska (usually .mkv) files.

Split a large .mkv file

Split to parts of fixed memory size

Split a 5.3 GB file MKV file large.mkv into 6 parts (part-001.mkv, part-002.mkv, …), each of which is no larger than 1 GB:

mkvmerge -o part.mkv --split 1G large.mkv

Split to parts of specified starting and ending times

Split a 2-hour video long-video.mkv into 2 parts (part-001.mkv, part-002.mkv) at the 55 minute of the video time 00:55:00:

mkvmerge -o part.mkv --split parts:00:00:00-00:50:00,00:50:00-02:00:00 long-video.mkv

Add subtitle file into a .mkv file

Check the ISO639 language codes:

mkvmerge --list-languages 

Add Enlgish-language subtitle file to a video file:

mkvmerge -o video_with_subtitle.mkv video.mkv --language 0:eng subtitle.srt