hi all - am struggling revising some docs at my new job so could use some help please.

When I click on a word to revise in the document, it highlights the full area in gray. If I space back to make a simple edit, it deletes the entire line. Can you please tell me how to fix this?

I'm using word 2016 but some of these docs are old so it's possible they were not made with 2016, not sure if that matters.