I changed the title in your question to give you a better chance of getting help.
How to post a question on this MSOffice forum

Do you have tracked changes turned on? If so, turn it off and accept all changes unlell you are using it deliberately.

Red lines and suggestions sounds like it might be words marked as spelling errors.
If you have red squiggly lines under words, that indicates that Word things that the word is not spelled correctly. When this happens for many correctly-spelled words, it means that the setting for your proofing language is wrong.
Click in one such word and then look at the language in your Status bar.
Which language is picked?
If the wrong language, try Ctrl+A to select all text. Then click on the language and pick the correct one.

Proofing language is tricky. If this is the source of your problem, write back for more help.
Spelling Checker Does Not Work! - Proofing Language Keeps Changing - Solutions