Aegisub

This worked well on 3.2.2 Ubuntu 20.10. Recommended.

First import video with: They don't have an aegisub executable without the version number. Amazing.

If you already have a subtitle file that you want to edit, then just pass it on as well:

Ctrl + P: play and pause video.

Ctrl + 3: set current substitle start time.

Ctrl + 4: set current substitle end time.

Enter: finish editing the current entry and start a new one.