One letter makes all the difference:

Affect and effect are frequently confused and I wish I had a dollar, or better still a pound, for every time I have seen them wrongly used. A good way to understand the difference is to remember that affect is normally a verb and effect is normally a noun.

Affect means to influence or change, while effect means the result, consequence, or change that is made:

• How does population growth affect us? verb
• The whole city was affected by the aftershocks of the earthquake. verb
• What is the effect of population growth? noun
• The effects of the earthquake were felt for many years. noun

But after this straightforward explanation, I have to break it to you that there are some cases which don’t follow the rules above. That’s English for you – just when you thought you were home and dry, the exceptions and differences come sneaking in round the door. So what about those rarer meanings that don’t follow the rules?

Affect can be used as a noun when talking about the mood that someone appears to have:

• She displayed a happy affect. noun

Psychologists find the word useful because they know that you can never really know what someone is feeling; you can only know how someone appears to be feeling.

Effect can be used as a verb which means ‘to bring about,’ or ‘to accomplish ’:

• He hoped to effect a basic change in policy. verb