How to add subtitles to a video using Aegisub, which is free and open source for Windows 10, Windows 7, Windows Vista, macOS, Mac OS X, Linux, Ubuntu, Debian, Linux Mint, Arch Linux...
The latest stable versions are at http://www.aegisub.org/downloads/ (If down, try https://github.com/Aegisub/Aegisub/re... )
Development builds are at http://plorkyeran.com/aegisub/
I switch from dev to stable mid-video but it wasn't necessary, I simply confused myself and blamed the software ^^