SRT subtitles are one of the most common subtitle formats accompanying DVDs and digital videos, like MP4s (MPEG-4s) and AVIs. Because Apple QuickTime does not natively support subtitle playback with MP4 video, you will need to download and install two QuickTime components: Perian and QuickTime Pro. Perian is the only component that supports SRT playback while QuickTime Pro allows you to import text files and add them to your video. As of this writing, QuickTime Pro costs $29.99 while Perian is a free, open-source QuickTime component.
Installing QuickTime Pro
Open your Internet browser and visit Apple QuickTime Pro's download page. Click the "Buy For Mac" or "Buy For Windows" button, depending on your operating system. Click the "Buy Now" button on the right side of the screen and enter the appropriate information, including your email address. Follow the on-screen instructions to purchase the QuickTime Pro registration key.
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Check your email for the 20-digit QuickTime Pro registration code sent from Apple. Highlight your registration code, right-click and select "Copy."
Open your current version of QuickTime Player and navigate to "Preferences." Right-click the registration code field and select "Paste" so your code appears in the field provided. Click "OK" or "Register" to automatically update QuickTime Player to QuickTime Pro.
Opening SRT Files in QuickTime
Open a new tab or window in your Internet browser and visit Perian.org. Click the "Download" tab in the middle of the page and click "Download Perian."
Double-click Perian's ".dmg" file when your download is complete to unarchive its contents. Double-click the "Perian.prefPane" icon to complete its setup.
Open your computer's System Preferences by clicking the "Apple" icon in the top left corner of your screen and selecting "System Preferences." Click the "Perian" icon at the bottom of the panel and click the "Load External Subtitles" button so that a check mark appears. Exit out of System Preferences.
Navigate to the desired MP4 file on your computer via Finder. Open a new Finder window and locate the SRT file. Rename your SRT file so the name exactly matches the MP4. For example, if your file is called "video1.mp4," name your SRT so it reads "video1.srt."
Drag and drop your SRT file into the same folder as your MP4.
Right-click your MP4's icon and choose to open it in QuickTime. Click the "View" tab, hold your cursor over "Subtitles" and choose "Track 1" to view your SRT subtitles over the video.