Affect is almost always used as a verb. It means “to change or make a difference.” How will this assignment affect my grade? (Think: How will this assignment change my grade?)
Effect is almost always used as a noun. It means “a result, impact, or consequence.” This assignment will have a major effect on your grade. (Think: This assignment will have a major impact on your grade.)
Remember those two rules above, and you’ll be right 99% of the time. Now here’s where things get tricky.
Affect is a noun if it means “emotion or desire.” Teachers tend to use this word to describe a student’s emotion based on the expression on his or her face. His affect told me that he was confused.
Effect can occasionally be a verb. In this case, it would mean “to cause something to happen or come into being.” I tend to think of this in terms of laws being created and voted upon. We must effect some new laws to protect our citizens. (In this sentence, old laws aren’t being changed; new laws are being created.)
Photo taken from here.