How to merge a subtitle file with an MKV video (fast way)
This tutorial will show you step by step how to merge SRT or SUB and IDX subtitle files with an MKV video file permanently.
You can also merge multiple subtitle files with an MKV video file.
How to merge subtitles with an MKV video using MKVToolNix
MKVToolNix is a set of free tools to create, split, edit, mux, demux, merge, extract, and inspect MKV video files.
It's available for Microsoft Windows, Linux, and macOS.
You can install this tool on your computer, or you can use the portable version.
You can download MKVToolNix here (official website).
The steps below will show you how to permanently embed (hardcode) the subtitles into an MKV video file.
1. Make sure that the subtitle file is in the same folder as the MKV video file.
2. Make sure that the subtitle file has the same filename as the MKV video file.
Example:
- Movie.mkv
- Movie.srt
- Movie.sub
- Movie.idx
3. Open MKVToolNix.
4. Click on the + Add source files button located at the bottom.
A “Add media files” window will appear.
5. Within this window, you go to the location on your computer where the MKV video and subtitle file(s) are.
6. Select the MKV video file and subtitle file(s) (.srt, .sub or .idx).
7. Click on the Open button.
8. In the Destination file section (located at the bottom), you can see and change the location on your computer where MKVToolNix will save the new MKV video file.
You can change the location by clicking on the 
(folder) button located next to the “Destination file” field.
9. When you're ready, you click on the Start multiplexing button at the bottom.
MKVToolNix will now merge the SRT or SUB and IDX subtitles with the MKV video file.
Read more articles about Windows
Related:
How to merge subtitles with any video permanently (4 ways)