How can I extract or join subtitles from/to MKV files?

I use MKVToolNix, its freeware opensource. Also search for MKVExtractGUI. If you install ToolNix and then extract the MKVExtractGUI zip file into the toolnix directory, you can use it to open and extract anything from the MKV container. If you want shortcuts on your desktop for the toolnix extractor or maker (mmg.exe), you can make them yourself by using a right click send to desktop.
In the newer toolnix there is an extractor already but the latest version is MKVExtractGUI2.exe and I add that manually for my shortcut. Also mmg.exe is an MKV maker but when you use it to view an already made MKV you get to see info that the extractor doesn't show, like what languages the audio and subs are. So use mmg.exe to see what you want (audio 1 subtitle 1 etc) then use the extractor to remove them. Also, if your MKV has normal audio and video (divx xvid and mp3 or ac3 audio) you can extract those separately and then open the video with the free Virtual Dub Mod and import the audio and V Dub will mux them together for you, set the video to direct stream (no encoding just copy) and save as an avi. If you want the sub in there too, its more work if you want it inside the avi (vobsub), or just make sure the name is exactly the same as the avi and keep them together and most players will pick it up.