Originally Posted by Usta
If you want to add subtitles, then add them here. If you do, then check "Burned In" box so that you don't have issues with most of the players.
I'd like to clarify that handbrake lets you hardcode (burn-in) external files in .srt format only. If you want to hardcode subtitles in the idx/sub format, you first have to embed them in the same mkv as the video using mkvmerge (it's quick); you cannot hardcode idx/sub subtitles stored as external files, at least with the current version.
This is the solution I use to play videos with DicePlayer on my Android.
If you only have an avi (not mkv) with idx/sub subtitles, you can also use virtualdub with the vobsub plugin to hardcode the subtitles.
Hope this is useful.