HandBrake

See https://www.makeuseof.com/tag/how-to-add-subtitles-to-a-movie-or-television-series.

1. Linux

2. macOS

3. Windows

3.1. Installation

Note: Handbrake requires .NET Desktop Runtime 6.0.1.

3.1.1. .NET Desktop Runtime 6.0.1

Run the windowsdesktop-runtime-6.0.10-win-x64.exe file with administrative privileges.
Click Install.
Click Close.

3.1.2. HandBrake

Run the HandBrake-1.5.1-x86_64-Win_GUI.exe file with administrative privileges.
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4. Usage

4.1. Add hard subtitles

Start HandBrake.
Select File | Open a single video file.
Select a video file.
Click Open.
Select the Subtitles tab.
Select Tracks > Import Subtitle.
Select the .srt subtitles file.
Click Open.
Check Burn In.
Click Browse.
Select the folder that contains the original video file.
Add «English, Subtitles» to the original file name.
Change the file extension from .m4v to .mp4.
Click Start Encode.
Wait several minutes for the encoding to finish.
Close HandBrake.

4.2. Convert from mkv to mp4.

Start HandBrake.
Select File | Open a single video file.
Select a mkv video file.
Select the Subtitles tab.
Click Selection Behavior.
Select Track Selection Behavior | First Matching Selected Language.
Select Available Languages | English.
Click Move Right.
Uncheck Add Closed Captions when available.
Uncheck Add Foreign Audio Scan.
Select Burn-In Behaviour | None.
Click Save.
Select 1 Unknown [UTF-8]
Check Burn In.
Click Browse.
Select the folder that contains the original video file.
Change the file extension from .mkv to .mp4.
Click Save.
Click Start Encode.
Wait several minutes for the encoding to finish.
Close HandBrake.