Affect 是個動詞，主要意為「影響；對…產生作用」，而 effect 通常用作名詞，意為「結果；影響；效果；效力；作用」。例如：
- Poor living conditions significantly affect educational achievement. (生活條件差會嚴重影響學業成績)
- The historic meeting will probably have the effect of boosting the government’s popularity. (這項歷史性會談可能有提升政府支持度的效果)
- Smoking will greatly affect your health. (抽菸將嚴重影響健康)
= Smoking will have a great effect on/upon your health. (抽菸將對健康產生嚴重影響)。
然而，effect 還可當動詞用，是個十分正式的 (及物) 動詞，意為「使產生，引起；實現，達到…的效果」，相當於 to bring about 的意思。所以，在 to effect change (使產生變化)、to effect a solution/settlement (解決)、to effect a reconciliation (調停，調解) 和 to effect a cure (治癒) 等片語中，正確的用字應該是 effect。有人可能認為 to affect change 也有意義，但它的意思是「影響變化」，亦即對變化造成影響 (to have an effect on change)，與「使產生變化」(to bring about change) 的意思並不相同。例如：
- Democracy effected great change in the lives of the people. (民主使人民的生活產生重大變化)
- He effected certain changes in government policies. (他使政府的政策產生某些變化)
- Japan was able to effect a reconciliation. (日本成功地進行了調停)
- He failed to effect a settlement of the dispute. (他未能解決這項爭端)。
- Such drugs can effect radical cures. (這些藥物可以達到根治的效果)
Affect 還有第二個、但比較少用的動詞意思，那就是「假裝，佯裝，裝作」，也是個十分正式的 (及物) 動詞。例如：
- You might affect surprise when someone gives you a gift you knew they would be giving you. (若你知道有人會送你禮物，那麼當他們送你禮物時，你可以假裝很驚訝)
- Much to my surprise, Paul affected not to see me. (令我感到非常驚訝的是，保羅竟假裝沒看見我)