Microsoft Word offers a grammar check feature that can help you check your writing for common mistakes. You can even use the Editor browser plugin to help you check for errors in your discussion posts (or social media!).
Google Docshas a good grammar check feature, as well.
If you'd like to brush up on grammar basics in more detail, explore the Writer's Reference Center, a writing self-help tool that includes a Grammar & Punctuation section. Find the Writer's Reference Center linked in the library's RESOURCES menu.