If your sentence is too long, it hampers the readability of your video. Splitting your sentences helps your video communicate better.
You can split sentences only on the 'Storyboard' screen. So make sure you split your sentences before you go to the advanced editing stage.
Here are the steps:
Step 1: Place the cursor at the point where you want to split your sentence.
Step 2: Either press 'Enter' on your keyboard or click the 'Split' button on the screen.
Here is a quick video to walk you through the process.