The importance of grammar can be seen in simple sentence construction. It can drastically influence the way your audience reads and understands even basic sentences in your essay. On the GRE Test, your score on the essay section will in part be determined by how well you construct sentences, and whether you correctly use grammatical components. According to the ETS website, essays that receive high marks will demonstrate facility with the conventions (i.e., grammar, usage and mechanics) of standard written English, but may have minor errors. So, you are not expected to submit a perfectly polished essay, because that requires more time than you are given. But, that doesn't mean that your essay can be plagued with a sleuth of grammatical flaws.
The following is a brief review of basic punctuation. Grammar mistakes are most often made when ideas become too complex. Keeping your sentences simple and clear will help you avoid getting tangled in difficult grammatical maneuvers. However, when you do write compound or complicated sentences, then correct usage of grammar becomes even more important, because then it is crucial that your ideas are well presented, and that your sentences are well structured.
The period is the easiest punctuation mark to use. It occurs at the end of most sentences. It is also used to identify an abbreviation. Some examples:
This sentence ends now.
"I'll wait and see," said Sarah.
In the above example, note that the period is at the end of the sentence, not the end of the quote.
Abbreviations: Mr., Mrs., etc., Dr.
Comma use is a little more complicated than the period. A common instruction for using a comma is to indicate a pause in the sentence. While this sometimes works, it is not an appropriate method to determine comma placement because it does not take into account the rules of use.
Sentences that are independent but joined together by and or, so, yet, but, for, and nor should be separated with a comma. For example, consider the following:
Most dogs will definitely eat human food, and a few will climb onto the table to get it.
If the first independent clause is very short, the comma can be left out. For example:
Our dog eats and often he eats way more than his share.
Commas should also be present in sentences with afterthoughts. For example:
Tim O'Brien then wrote 'In the Lake of the Woods', probably his best novel.
Another example is comma use in pairs to serve as less obstructive parentheses:
The best examples of comma use are, I think, presented in the most common language.
The author's ideas, first seen in early philosophical writings, are not original to his text.
Comma use in lists is the most common and easy to identify. Typically a comma is used between each item in the list. However, on the GRE test, when you are writing your essays, you'll want to refrain from the use of lists. Rather than enumerating items one-by-one, explain them in detail.