Top Ten List of Common Grammatical Errors
10. The use of sexist language. Gibaldi states in the MLA Handbook, "Because good scholarship requires objectivity, careful writers . . . avoid language that implies unsubstantiated or irrelevant generalizations about such personal qualities as age, economic class, ethnicity, sexual orientation, political or religious belief, race, or sex" (37). In other words, don't use language that could be deemed insulting or ignorant.

9. Do not change verb tense within a single paragraph.

8. Use the active voice in all your writing. It will make your paper that much more exciting to read. For example, "The reason he left 220 was that his health was impaired," is a passive sentence. The sentence becomes more vigorous in the active voice: "Failing health compelled him to drop 220." Notice that it is also a shorter sentence. Which brings us to point number 7.

7. Never use more words than necessary. This doesn't mean that you should abandon detail and color, spitting out short, staccato sentences like watermelon seeds, only make sure that every word counts. "Very," for example is a fluffy word that is only RARELY needed. Another overworked and mushy phrase is "the fact that." In all situations this phrase can be transformed into a tighter package, for example:
owing to the fact that since or because
in spite of the fact that though or although

I suggest that you revise "the fact that" out of every sentence.

6. The way to form a possessive singular of nouns is to add - 's- to the end no matter what the last letter is, thus,
Marx's precepts
Burns's poem
the witch's broom.

Plural nouns use only the - ' -:
Vanderbilts' estate
the Woodsons' crazy dog.

You do not, however, use an apostrophe to form the plural of an abbreviation or a number:

PhDs
1990s.

5. Do not mistake its for it's. The first is a possessive, the second is a contraction of "it is." It is best, actually to spurn all contractions in formal language. Just say no.

4. Plays are considered literature and thus are always happening. You write about them, then, in the present tense. Productions of plays are ephemeral and singular. You write about them, then, in the past tense.

3. All parts of MLA style papers are double spaced including the works cited and off-set quotes. Margins are one inch and you should use font no bigger than twelve pica and no smaller than ten.

2.Just because you spell check does not mean the word will be spelled correctly. When in doubt, look it up. The moral of this is PROOF READ! PROOF READ! and then PROOF READ! Try reading your work backwards sentence by sentence, this can help you look at the words and not automatically move into meaning.

1. Play titles are ALWAYS underlined or italicized.

More at: http://herbergeronline.asu.edu/the220/notes/notes.html