Why has spell/grammar check become so ineffective?

The AI bot answer has given you a laundry list of controls for the spelling checker. I doubt that is what you need.

No one here can answer "why" questions about Microsoft decisions. We are your fellow users. They do not tell us.

That said, as far as I can tell, the spelling and grammar checkers are working fine in Microsoft 365, at least as well as they have for the past twenty years. Perhaps if you give us more detail about the problems you are seeing, we can help you.

https://learn.microsoft.com/en-us/answers/questions/5677122/(article)-how-to-ask-a-question-in-the-q-a-forums

You may want to give us examples of the problems and perhaps even save a sample document on OneDrive or DropBox and post a share link here.

Suggestion:

Try clicking on an obviously misspelled word that has not been marked by Word as a problem.

Check the language setting for that word by clicking on the language in the Status Bar or by going to the Review tab and going through the language settings.

Is "Do not check spelling or grammar" checked?
If it is, exit out of the language window and select all the text in your document (Ctrl+A). Then check the language settings again and clear the box for "Do not check spelling or grammar." Then run spell-check again.

Excellent resource: Mastering the Spelling Checker by Word MVP Suzanne Barnhill:

http://wordfaqs.ssbarnhill.com/MasterSpellCheck.htm

I prefer the older spell check dialog to the Editor version. See:

• https://addbalance.com/word/spell_check_dialog_instead_of_editor.htm#pagestart
• https://addbalance.com/word/download.htm#LegacySpellCheck

Dear @Sean Powers,

Thank you for reaching out to Microsoft Q&A forum.

I’m truly sorry for the inconvenience you're experiencing with Word’s proofing function. As a forum moderator, I genuinely wish I could directly access your account or delve into the backend systems to diagnose and fix this for you. However, our role here is limited to providing general guidance and solutions that can be applied by users. 

From my research, Microsoft has shifted Word's proofing engine from a traditional rules-based system to a cloud-based AI "Editor". This means that instead of looking for strict dictionary matches, the software now attempts to predict "natural" or "concise" language, which often leads to the informal or incorrect suggestions you are seeing.

If you find the current AI suggestions ineffective, you can revert to a more traditional proofing experience or fix the underlying issue where Word ignores legitimate mistakes. Here are a few steps that might help:

1.Disable AI Refinements

To stop receiving "text-style" or grammatically questionable suggestions, you can configure Word to focus on basic spelling and grammar.

• Go to File > Options > Proofing.
• Under When correcting spelling and grammar in Word, ensure Writing Style is set to Grammar (instead of "Grammar & Refinements").
• Click Settings next to Writing Style.
• Uncheck everything under Clarity, Conciseness, and Formality. Word will stop trying to "improve" your tone and only flag technical mistakes.

2.Fix "Missed" Spelling/Repeat Words

If Word is missing obvious mistakes (e.g., "the the"), it could be due to issues with your Proofing Language or the Normal.dotm template.

• Highlight your entire document (Ctrl + A).
• Go to the Review tab > Language > Set Proofing Language.
• Ensure "Do not check spelling or grammar" is unchecked. This issue is often caused by this box being partially filled.
• Click Set as Default to ensure this applies to all new documents.

3.Reset the Custom Dictionary

If Word has "learned" certain mistakes over time, it might ignore legitimate errors that were accidentally added to your personal dictionary.

• In File > Options > Proofing, click Custom Dictionaries.
• Select RoamingCustom.dic and click Edit Word List.
• If you see common typos or repeated words in this list, delete them. Word will be forced to re-evaluate those words against the standard dictionary.

I hope this information is helpful. Please follow these steps and let me know if it works for you. If not, we can work together to resolve this. Thank you for your patience and understanding. If you have any questions or need further assistance, please feel free to share them in the comments so I can continue to support you. I'm looking forward to your reply.

If the answer is helpful, please click "Accept Answer" and kindly upvote it.

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Word’s spelling and grammar checking is handled by Microsoft Editor, which can be tailored or limited, but not fully reverted to an older “classic” behavior. On the user side, the only available options are to adjust what Editor checks, which languages it uses, and to turn categories off if they produce low‑quality suggestions.

To improve or “fix” proofing behavior in Word for Microsoft 365 on Windows:

1. Confirm Editor is the feature in use
   • In Word, go to Home > Editor.
   • The Editor pane will show categories such as Spelling, Grammar, and various Refinements (Clarity, Formality, etc.).
2. Turn off suggestion types that are not useful
   • In the Editor pane, select a category (for example, Clarity or Formality).
   • For suggestion types that are consistently unhelpful, choose Stop checking for this so Editor no longer flags them.
   • This reduces “text‑message style” or overly casual suggestions and keeps only basic spelling/grammar if desired.
3. Tailor what Editor checks for
   • Select the text, then go to Review > Language > Set Proofing Language and choose the correct language.
   • This ensures Editor is using the right language rules and can reduce incorrect suggestions.
   • When multiple languages are present, the Editor pane will list issues per language so they can be reviewed separately.
4. Use the Editor pane instead of only in‑line underlines
   • On the Review tab, select Editor.
   • Step through Spelling and Grammar issues using the arrows at the top of the pane.
   • Choose Ignore once for bad suggestions; if a whole category is unhelpful, disable that category as above.
5. Check for configuration or proofing issues if errors are missed entirely
   • If misspellings are not underlined or not found at all, there may be a configuration problem rather than a quality issue. Common causes include:
     • Proofing tools not installed.
     • The speller add‑in for the language (for example, EN‑US) disabled.
     • Do not check spelling or grammar enabled for the text.
     • A different default language set.
   • These issues can cause Word to skip errors completely.
6. Use the classic spell check pass when needed
   • Press F7 or go to Review > Spelling & Grammar to run a full check.
   • This can sometimes catch issues differently than relying only on in‑line suggestions.

If, after tailoring Editor (disabling refinements, ensuring the correct language, and verifying proofing tools), the results are still unsatisfactory, Word does not provide a built‑in way to switch to an older proofing engine. At that point, using another editor in parallel is the only user‑side alternative.

References:

• Check grammar, spelling, and more in Word
• Check spelling and grammar in Office
• Check grammar, spelling, and more in Word for the web
• The spelling and grammar checker isn't working as expected
• Spell Check does not work in Word 2010
• Check grammar and spelling with the Microsoft Editor browser extension