It will make a very slight difference to SEO as keyphrases are often correct grammar unless you would usually include an apostrophe, people don't tend to use those in searches. Do research first though, make sure there is no popular mixture of keywords that doesn't make sense.
Google is a semantic serach engine, so it looks a phrase relationships. It usually better to SEO in your own language as you have a better understanding of keywords, phrases and how these relate to your products and services.
Probably it doesnt affect SEO as Coolvision thinks, but always keeping in mind to what login said. Anyway a bad grammar makes your site look non-proffesional=less traffic in the future~!!
when the KW is the one misspelled, it can affect your SERP as the missspelled word is the one which SEs added weight.. "celebrity is different from the mis spelled celebrty" When it comes to normal text I think that's ok but the readers and visitors of your site might irritate with this one
Yep, misspelling makes your searches different results. If you are expecting a good or exact result, you should also consider the correctness of your grammar. So grammar would affects SEO.