Word's spelling and grammar checker is a great tool, but it is far from perfect. The system often flags "mistakes" that are actually correct. To prevent Word from flagging an item in the future, you can simply click the Ignore or Ignore All button in the Spelling And Grammar dialog box while you're running the spelling checker. Or, if the spelling and grammar checker isn't running, you can right-click ([control]-click on the Mac) on a flagged word or phrase and select Ignore or Ignore All from the resulting shortcut menu.
When you set a word or phrase to be ignored by the spelling and grammar checker, it is ignored if you run the spelling and grammar checker later, whether the spelling and grammar are correct or not. If you want to change your mind and want ignored text to be checked instead: First, select Tools/Options (Preferences on the Mac) from the menu bar, and then click on the Spelling & Grammar tab. Next, click the Recheck Document button at the bottom of the dialog box.
When prompted to continue, click Yes. This removes the "ignored" status from any items you chose to ignore during an earlier spelling or grammar check. Click OK to close the Options (or Preferences) dialog box, and then run the spelling and grammar checker to recheck the document.
Note: In order for the technique described above to work correctly, Word must be configured to check spelling and grammar as you type. To check Word's spelling and grammar settings, select Tools/Options (Preferences on the Mac) and click on the Spelling & Grammar tab. Make sure the Check Spelling As You Type and Check Grammar As You Type check boxes are selected. In addition, make sure that the Hide Spelling Errors In This Document and Hide Grammatical Errors In This Document check boxes are cleared. When you've finished, click OK.