You’ll notice that your file is saved, by default, in the Advanced Substation Alpha (.ASS) format. Just click File > Save or the Save icon in the menu bar. When you’re done subtitling your MP4, you’ll want to save the subtitle file so you can use it later. Be sure to play the whole video from time to time to make sure that your subtitles are timed well. Once you’ve created a subtitle, just repeat the process for the rest of the audio in your video. If you need to manually create a new subtitle, just right-click in the subtitle grid and select Insert (before), Insert (after), Insert at video time (before), or Insert at video time (after). When you commit changes to a subtitle, Aegisub will create a new selection for you that you can then use for the next subtitle. If you need to tweak the times that the subtitle is displayed, just click the subtitle in the grid and change the start and end times in the subtitle editor below the audio track.