Loading an IDX file while playing a movie on your computer allows your media player to display subtitles during playback. To display subtitles during a movie, you need both an IDX and a SUB file. While the SUB file contains the actual subtitles, the IDX file synchronizes them with the movie, so that they appear at the right time. You can run IDX and SUB subtitle files with Windows Media Player, an application you can find in all recent versions of Microsoft Windows.
Click on the "Start" button on your task bar, and select "Computer" in the Windows Start menu.
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Locate the IDX and SUB files you want to use, and, drag them to the same folder of the one containing the movie you want to play.
Click once on the IDX file and rename it so that it has the exact same name of your movie file. For example, if your movie file is "Apples.avi," change the name of your IDX file to "Apples.idx". Do the same for the SUB file.
Right-click on your movie file, and select "Open with Windows Media Player."
Open the "Play" menu, and select "Lyrics, Captions and Subtitles." Select "On if available." Windows Media Player will now display the subtitles contained in the SUB file and synchronize them by using the IDX file.