How to insert text into a sentence?

PLEASE don't edit text if you are asked to review. Not for big changes or small, this is not what you are expected to do and could be disastrous. In the worst case the publisher will check for marked changes, find none, and assume you have approved everything - it will go to press uncorrected.

Let's take a step back to the world of paper. An author would be sent a printed version of what the publisher had typeset. The author would write on the paper in a very specific way, using special symbols meaning "insert text", "delete text" or "edit text". And this is exactly what you need to do with PDF, but using tools in Acrobat. Don't guess, don't try to make it easier for the publisher, do it in this one way only, because the process might actually be automated, and if you do it differently, your changes won't be seen.

https://www.smith.edu/its/tara/software/documents/AdobeCommentandMarkupTools.pdf is a useful file. But it also includes some tools you should not use. Go on page 2 to "Text editing" and read about "Delete text", "Replace text" and "Insert text". Use it for everything from a rewrite to a new full stop.

Have you been asked to review a document, returning the corrections to an author or designer?

Or are you trying to change a file directly, to be delivered to your readers?