Affect versus Effect

“Affect” is a verb and “effect” is a noun.  What does this difference mean in your writing?

Affect

If someone or something is performing an action on another person or thing, use “affect.”  “Affect” often has the sense of “to influence”:

• My partner affects the way I behave: she encourages me to be a better person. (“My partner” is performing an action on me.)
• The rain affected driving conditions: it made the road slippery. (“The rain” is performing an action on the road.)
• The medication will affect you in two ways.  It will make you drowsy and hungry. (“The medication” is performing an action on you.)

Effect

“Effect” is a thing.  It often has the sense of “result.”  You can count the number of “effects” just as you can count any other thing.  “Effect” will therefore often (but not always) have one of the following types of words before it:

• an article like the or an;
• a counter like one, two, three, etc.;
• a possessive adjective like my, your, his, her, its, our, or their;
• an adjective that describes the effect like harmful, clear, or negative; or
• a word like many, some, few, no, etc.

We can see such words before “effect” in the following examples:

• My partner has a positive effect on the way I behave.
• Rain has an effect on driving conditions.
• The medication will have two effects on you: it will make you drowsy and hungry.

Resources

Grammar Girl has a good tutorial on the difference between affect and effect.  

You can practice using “affect” and “effect” in the online quizzes on Grammar Book, Grammar-Quizzes, and The Scriptorium.