Poor grammar doesn’t just hurt SEO it also hurts user experience. Nobody wants to read content that makes little or no sense. Keep in mind that depending on what you are selling your content will build trust in the eyes of your website visitors. If your content is poorly constructed your audience might lose trust in your company and venture over to a competitor. Do you really want that?
Google Video discussing SEO and grammar
Here is a great infographic by the folks at CopyBlogger discussing some important grammar mistakes you might be making.