The following are errors that I see frequently in student papers--avoid them!
Misuse of the Apostrophe
I've come to pity the apostrophe because I so often see it neglected and abused. This is by far the most common error that I see in my students' writing.
Tip: The apostrophe is most often used to show possession or ownership, so try re-wording the sentence to use an "of" phrase--for example, "...the desk of the teacher..."--in order to figure out where to place the apostrophe. The apostrophe immediately follows the owner's name ("the name of the owner"--see how easy that is?), and is then followed by an s if the owner's name does not already end in s.
Example: ...the teacher's desk... (if it belongs to one teacher)
Example: ...the teachers' desk... (if it belongs to two teachers)
Tip: "Affect" is most often a verb indicating the influence that one entity has over another entity; "effect" is most often a noun representing the outcome of that influence.
Example: Motivation often affects grades in school.
Example: The effect of motivation on grades is often quite easy to observe.
Tip: Use "amount" when there is no practical way to count whatever is being described; use "number" when it is possible to count the thing(s) being described.
Example: The amount of time it took was more than the teacher had estimated.
Example: The number of students in the classroom exceeded the number of desks.
Tip: "Contribute" means giving a donation of money, effort, etc.; "attribute" (as a verb) means recognizing the source of success, failure, etc.
Example: I will contribute to the fund for the American Heart Association.
Example: I attribute my success in school to my dedicated efforts to succeed.
Tip: The word "data" is plural; the singular form (which you will probably never need to use) is "datum."
Example: These data suggest that students learned a great deal.
Example: The data were kept by the teacher for grading purposes.
Tip: "Everyday" is an adjective used to indicate a commonly encountered event. "Every day" is a phrase indicating that a given event took place on each day specified. To determine which of these forms to use, try saying "every single day" and if that makes sense in the context of the sentence, insert the space.
Example: Apostrophe mistakes are an everyday occurrence.
Example: We met every day to practice for our presentation.
Tip: "Farther" refers to physical distance; use "further" to refer to distance in time or any other non-physical distance.
Example: The more disruptive student sat farther away from the teacher.
Example: We could not go any further into the lesson due to lack of prior knowledge.
Tip: This is the same idea as in the amount/number distinction--use "fewer" when it is possible to count the items, and use "less" when the items cannot be counted.
Example: There were fewer students in the hallway today than there were yesterday.
Example: We have less time to complete the work than we were originally told.
Tip: Use "principle" when you are referring to a rule or guideline; use "principal" when you are referring to the primary player (e.g., a school principal).
Example: The principles given on this page should help students to write better.
Example: Max got sent to the principal's office.
Tip: "Than" is a word indicating a comparison; "then" points to a specific time.
Example: Joe had an easier time with the homework than Bill.
Example: Ms. Strict handed out the rewards and then distributed the punishments.
Tip: "Their" is a possessive pronoun; "there" often indicates a specific place.
Example: The students took their work home with them.
Example: The students had a good time over there.