Affect and effect are often mixed up. With only one letter difference, it can be confusing. However, if you remember that affect with an a is a verb and effect with an e is a noun, it helps immensely.
Affect means “to influence,” so if you ask yourself when writing if this is the meaning you want, you’ll probably use the proper word.
The book affected me deeply.
Effect is like a result. If this meaning fits what you’re writing, use the e.
The effect was dazzling.
Of course, like most things, there are exceptions to this rule.
If you are using affect in regards to psychology, you can use it as a noun. It is often used in this way to describe a mood in psychological terms.
Her face reflected a sad affect.
Likewise, effect can be used as a verb in special cases when you want to “bring about” something.
The mothers hoped the nurse-in would effect the change they wanted.