There are various free tools available on Google nowadays to check any error. For grammar, the app which is mostly used nowadays is Grammarly, there is a premium version available but the free one works just fine. Another website for grammar is a small SEO tool, this works great, it points out every mistake with the right answer. Moreover, this website is good for plagiarism checkers as well. Just copy the content and paste it into the box and you have the results.