|Big picture: if you are "New to coding," I think pursuing something this complex ... inter-op with Word ... is not a good strategy to maximize what you learn. Why not take one of the many extended Rich TextBoxes for WinForms, and use that for your editing control ?
To tango with Word, you need to research, and learn to use, inter-operation between MS Word and C#. Lots of resources on this: search here, and on the net [^]
Then, you need to determine what are the possibilities for intercepting Key Events in MS Word from C#: can you install/hook your own KeyDown handler from C# into the running instance of MS Word ? , [^], For all I know, this may require writing a Word add-in.
If you get that far, then you have to implement your own equivalent to an auto-completion facility: that's not hard: if you ask a specific question on that, I will respond with code.
If you can't get MS Word key events in C# as described above: then you are going to need some form of hook, as Dave K. mentions.
If you do get a hook working, then every keystroke in Word is going to require a call though the hook to your code: I suspect that will have a real impact on performance, depending on your hardware, memory, etc.
«... thank the gods that they have made you superior to those events which they have not placed within your own control, rendered you accountable for that only which is within you own control For what, then, have they made you responsible? For that which is alone in your own power—a right use of things as they appear.» Discourses of Epictetus Book I:12