LanuageTool is a FOSS freemium tool to help with your grammar checks online and in documents.

The application is a pretty good alternative to ProWritingAid and Grammarly. The Freemium features are more limited on the browser extension, but it is privacy centric in that you do not need to create an account. The extension for LibreOffice will find extra errors including a variety of grammar errors like passive voice, punctuation, and more.

On this video, I show you how to install LanguageTool on Firefox, Chromium based browsers (like Google Chrome). We also look at installing and configuring the options in LibreOffice. Note that with a paid account you can unlock extra grammar checks and the API will allow you to link your account to the latest version of LibreOffice.