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Poor grammar can be a spam signal to search engines. A typo here and there will not hurt you -- but major mistakes with spelling and grammar in every word and every sentence will likely hurt you. There's a middle ground somewhere, but only the search engines know what that is. Of course, I would copyedit and proofread so that all text is perfect. Not for search engines -- but so that users do not see poor spelling and grammar and then judge your website to be less quality and authoritative. In the end, it's all about usability for humans. Search engines generally aim to show what websites users think are the best.