In almost all cases, “affect” should be used as a verb, and “effect” should be used as a noun:
“A high SAT score can *affect* your chances of getting into the college you want, because it has the *effect* of making colleges rank your application highly.”
Part of the confusion with these two words comes from the fact that “effect” can be used as a verb in special cases, generally as part of the phrase “effect a change”:
“The student’s SAT tutoring is likely to *effect* a change in her chances of receiving a scholarship.”
And conversely, the word “affect” can be used as a noun in special cases to mean “the display of an emotional response”:
“One of the symptoms of severe depression is a flat *affect* — the failure to display emotion on one’s face in response to negative or positive events.”
Knowing the proper grammatical use of words can help you in daily life, in your professional life, and on the SAT, ACT, GMAT, and GRE. If you are planning to take any of these tests, contact me for more information!