Affect and effect are two English words with very similar meanings, and very similar pronunciations. Though not actually homophones, the sound of the words is similar enough to cause most people confusion. Even some people who use them quite often will find themselves stumped, and have to pause to think about which word is appropriate to use.

The word affect is primarily used as a verb in English. It has two main meanings, which are closely related. Affect may mean to alter the feelings of, or to change the mental state of someone or something. It may also mean, in a broader sense, to change or affect someone or something in any way. So it is that we might say: How will the election affect the course of history? We might also say: The painting affected him so deeply he could not speak.

The word effect, in contrast, is used primarily as a noun in English. It has a number of related meanings, but generally speaking refers to the result of something, the power something might have to get that result, or a phenomenon in the world. An example of the word used to refer to the result of something would be: The election had no effect on the course of history. We might also say: The effect of the painting on him was profound. Phenomena are often also referred to as effects, such as the photoelectric effect or the greenhouse effect.

Affect may rarely be used as a noun, but this usage is esoteric and somewhat archaic. In psychology one may speak of a person's affect, which refers to their mood or mental state. So we might say: The patient exhibited a flat affect, responding to no stimuli. More archaically, the word affect may be used as a noun to refer to an affectation someone might take on — a pose, or disposition. In this context we might say: Her affect was one of learning and grace, but those of us who knew her knew better.

Effect may also be occasionally used as a verb. When it is used in this way, it refers to something having a direct effect, or making it happen. It is usually suffixed by -ed, and takes an object. This use is somewhat more common than the noun uses of affect, but still nowhere near as common as its traditional noun form. An example of this usage would be: The election at last effected the change the people had been hoping for.

Although we have addressed the exceptions to the rule, generally speaking the word affect is used when a verb is called for, and effect is used when a noun is needed. A mnemonic device to help you remember this is that a verb is an action word, and both action and affect begin with the letter a.

An example sentence demonstrating the various uses of these words would be:

Although the painting seemed to affect him very deeply, its true effects would not be known until after the liquor — which had affected his judgment — wore off.