Did I just hear you groan? Good writing is basic to good filmmaking. The quickest way to lose all credibility as a filmmaker or writer is to have a screenplay full of grammatical and spelling errors.
There are two solutions:
1. Learn the rules of grammar and how to spell.
2. Find someone who is really good at spotting problem with grammar and spelling. (Remember making a film is a collaborative art.) Thank them profusely for their efforts and contributions to the film. Everyone serves some purpose in filmmaking.
The best solution is to do both 1 and 2.
A good basic resource is William Strunk, Jr.'s The Rules of Style. This short, concise volume has been a standard reference for almost 90 years. You can download it as part of the Complete Online Filmmaking Reference by clicking on this link: