Keyframing

What are keyframes?

• These are sudden and instantaneous transitions to a different scene. (e.g. Jump Cuts, Camera Switches)
  If you encounter one of these, the lead-out should be on the exact frame before the transition happens.

When there are already subtitles in a video:
You should try to match the keyframes of the hardsubbed subtitles in the video to your subtitles.
You can also try to mask the hardsub so you don’t have to match it 
but this requires editing software.