It is important to note that revising and editing are two different activities and are done at separate times.
Revising is what you do to your first draft after you write it. From a very basic point of view, you want to review the paper to ensure you:
1) answered the question (if you are writing an essay for a homework assignment, for example) or that your point of view is clear to the reader.
2) You want to make sure the reasons and evidence you are providing to support your point of view (or answer to your question) are also clearly stated . This is also a good time to make sure that sources are cited and credit is properly given to the source’s author.
3) Finally you want to make sure your paper is organized in a way that leaves the strongest impression on the reader.
Editing on the other hand, is done only after revising the content and organization of the paper. Again, from a very basic point of view, it is during editing that you check for consistent verb tense though out the paper, proper use of pronouns, commas and subject verb agreement, etc. The entire purpose of ensuring correct grammar is to make it as easy for the reader as possible to understand your point.
So the next time you have a paper to edit, be sure to break revising and editing into two steps. When revising, don’t worry about the grammar, just make sure the paper clearly says what you want it to say. When editing, keep your focus on grammar.