Is it affect or effect?
Usually, affect is a verb meaning to influence, and effect is a noun meaning a result. For example: The weather affects my mood. The weather has an effect on my mood.
A helpful memory trick is RAVEN: Remember, Affect is a Verb, Effect is a Noun. If you can put the before it, as in the effect, you probably need effect. If something is doing the influencing, as in rain affects traffic, use affect.
There are exceptions. Effect can be a verb meaning to bring about or cause, as in The new manager effected major changes, meaning she made changes happen. Affect can be a noun in psychology, referring to a person's visible emotional expression, as in The patient showed a flat affect.
Why it is confusing
The two words sound very similar in everyday speech, which makes them easy to mix up. Both come from Latin words related to doing or acting. Reading sentences aloud and checking whether you need an action or a thing can help you choose correctly.
Did You Know?
Special effects in films, often shortened to FX, use the noun effect, because they are results created on screen.