When you spell something incorrectly, Prezi will underline the word it doesn't recognize with a red line. This is just as you would see it in Microsoft Word or any other text editor.

To correct the word, simply right-click on it (or Command + Click on Mac OS) and select the word you meant to type from the suggestions, as shown in the following screenshot:

So a colleague of yours has just e-mailed you the text that they want to appear in the Prezi you're designing for them? That's great news as it'll help you understand the flow of the presentation. What's frustrating, though, is that you'll have to copy and paste every single line or paragraph across to put it in the right place on your canvas.

At least, that used to be the case before Prezi introduced the drag-apart feature in the text...