To Be Effective
To Work Effectively
intransitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means effective. The verb vocab version is to be effective or to work effectively (since "to effective" doesn't make sense).
This word is used to describe something that works effectively and as intended, such as medicine that achieves the desired effect or brakes that effectively stop the car.
What is particularly effective? It's your key (き). It's effective at opening your house door.
やっと薬が効いてきた。
The medicine is finally starting to take effect.
この温泉は肩こりによく効くらしいです。
I heard that this hot spring is very effective for stiff shoulders.
いや、ブレーキが効かなかったんだよ！
No, the brakes didn't work!
ようやく広告が効いてきたのかな？
I wonder if the advertisement has finally started to have an effect?
コウイチの昨日のお説教がマイケルに効いたみたいです。
It seems that Koichi's ear-bashing yesterday was effective for Michael.