Microsoft has its answer to Grammarly. The company recently introduced Microsoft Editor, its AI-powered writing assistant, which supports 20+ languages. It works as a browser add-on on both Microsoft Edge and Google Chrome. If you don't want to spend to use Microsoft Editor, there is a free version that offers basic features like checking spelling and basic grammar on Word, Outlook.com, and anywhere else on the web.
Now, if you have a Microsoft 365 Personal and Family subscription, you are offered more advanced grammar and style refinement suggestions that help you with things like clarity, use of formal language, conciseness, and vocabulary suggestions, among other things. The add-on is already available on the said app stores, so if you want to give it a try, you can do that right now.
Source: MSPowerUser