During playback, in iTunes or on the Apple TV, you can choose the subtitles the same way as I described above, unless, of course, they're burned in. In this case, they'll always appear. If you play back the video with other software, such as VLC, choose the Subtitles menu to set subtitles.
Subtitles from opensubtitles.org may not be perfect. As I said, these are crowd-sourced, and there may be glitches, or translations that are not as good as what professional translators may provide. But it's better than nothing.
If you want to add subtitles to movies you've already ripped, you can do that using Subler (donation requested). This app will allow you to add .srt files to existing videos.
Roll your own subtitles
An .srt file is simply a text file formatted in a specific way. You can create your own subtitles and add them to your movies, as described above. To do this, you can either use a text editor or a dedicated tool for creating .srt files. Using a text editor is complicated because you need to know the precise time codes.
Here's what .srt files look like. Each subtitle has a number, and its duration on screen is specified by time codes. Time codes are in the format hours:minutes:seconds,milliseconds.
0 00:00:00,000 --> 00:00:02,300 I can do it, I know I can!
1 00:00:03,533 --> 00:00:06,100 Look at me, I'm walking!
2 00:00:10,266 --> 00:00:14,667 It's moving too fast!
A great tool for creating and editing .srt files is bin liu's $10 SRT Edit Pro. This app allows you to see your video and an audio waveform (so you can tell when dialog begins and ends), and lets you insert time codes and subtitles.
Find where you want a subtitle to begin and click Insert TC. Type your subtitle after the time code and then, at the location where you want the subtitle to no longer be visible, click Insert TC again. Continue through your video. You can use the audio wave form to show you when dialog begins and ends. It's best if your subtitles correspond, as much as possible, to the timings of people speaking.
You can also use SRT Edit Pro to edit existing .srt files that you've downloaded, if you want to make corrections to them. Once you have your .srt file, you can add it to your video with Handbrake, as explained above.
If you're making your own subtitles, I have some tips that can help you make them easier to read. Back in the day, when I worked as translator, I translated a number of French movies for English subtitles. This was before the type of software that's now available. I was given time sheets with time codes and character counts and little else. The standard was no more than 18 characters per second, with no more than around 36 characters per line. A subtitle should never overlay a cut; in other words, if you have some dialog with shots that cut back and forth, you should try to make each of your subtitles fit with a single shot. You may need to compress the text by leaving out some words. (If you want to know more, the BBC has an excellent guide to subtitling for closed captions, but the principles are the same for foreign language subtitles.)
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