I have an mkv-file I want to re-code because my hardware-player have some problems with it.
The resulting code or file-format doesn't matter for me. Maybe a simple avi-file.
There is a subtitle inside of the mkv. I want to keep it in the new coded file. How can I do this?
Not really enough information to help out much.
If mkv is a problem then which multimedia container plays properly on your player?
Else you need to demuliplex to extract to subtitle stream and store it seperately.
What kind of subtitle file that will be depends on what is inside the MKV.
Also there are plenty of subtitle utilities that will let you change the subtitle format if needed.