Microsoft Word has spell checking enabled by default. However, if it's not working, go to File {'>'} Options {'>'} Proofing and ensure 'Check spelling as you type' is selected. This setting enables the automatic spell checking that underlines misspelled words in red as you type. For grammar checking, also make sure 'Mark grammar errors as you type' is checked. To perform a manual check of your entire document, press F7 or go to the Review tab and click 'Spelling & Grammar' (or 'Editor' in newer versions).

To add words to Word's dictionary, right-click on any word that Word has flagged as misspelled (with a red underline) but that you know is correct—such as a name, brand, or technical term—and select 'Add to Dictionary' from the context menu. Alternatively, you can manage your custom dictionary by going to File {'>'} Options {'>'} Proofing {'>'} Custom Dictionaries. This opens the Custom Dictionaries dialog where you can edit existing dictionaries or create new ones. Words added to the dictionary won't be flagged as spelling errors in future documents.

To check spelling in multiple languages in Word, first ensure the language packs are installed. Then, select the text written in a specific language, go to Review {'>'} Language {'>'} Set Proofing Language, and choose the appropriate language from the list. You can repeat this process for different sections of text in different languages. Word will automatically use the correct dictionary for each language-marked section. For automatic language detection, check 'Detect language automatically' in the Language dialog. Remember that each language must have proofing tools installed for spell checking to work.

Spell check in Word focuses primarily on identifying misspelled words and basic grammar issues. Editor, available in newer versions of Word (especially Microsoft 365), is a more comprehensive tool that goes beyond spell checking to provide suggestions for improving your overall writing. Editor checks for spelling and grammar errors but also offers feedback on clarity, conciseness, formality, inclusiveness, vocabulary, and readability. It's organized into categories like Correctness, Clarity, Engagement, and more. Think of Editor as an advanced evolution of the traditional spell checker, offering more in-depth writing assistance.

Word might flag correctly spelled words for several reasons: The word might be correct but not in Word's dictionary (especially technical terms, proper names, or industry jargon). The text might be set to a different language than intended—check by selecting the text, going to Review {'>'} Language, and setting the correct language. The word might be spelled correctly but used incorrectly in context (like 'there' instead of 'their'). Or Word's dictionary might be outdated or corrupted. For words that are correct but not recognized, add them to your custom dictionary by right-clicking and selecting 'Add to Dictionary' to prevent future flagging.