下がる and 下げる: fall, lower, or step back
Separate a position or value becoming lower from an influence that lowers it. Also learn where 下がる means stepping back and where another movement verb is more natural.
A position lowers or someone lowers it
Use 下がる when an object's position becomes lower. ボタンを押すと、ブラインドがゆっくり下がります。 says that pressing the button makes the blind descend slowly.
Use 下げる when someone lowers the object's position. 日差しが強いので、ブラインドを下げてください。 marks the blind being lowered with を.
Temperature, price, and other values
A value that falls takes が in phrases such as 気温が下がる and 値段が下がる. 夜になって気温が急に下がりました。 says that the temperature dropped sharply at night.
The value being lowered takes を with 下げる. 画面が明るすぎたので、明るさを下げました。 marks the adjusted setting with を. The cause need not be a person, as in 冷たい風が室温を下げた.
Step back with 下がる and reduce with 下げる
A person can use 下がる to move back from their current position: 危ないので、線の後ろまで下がってください。 For ordinary movement down a flight of stairs, 階段を下りる is usually more natural, so do not replace every “go down” with 下がる.
To speak more quietly, say 赤ちゃんが寝ているので、声を下げてください。 下げる combines with nouns for degree and level as well as physical position.
Quick check: Which is correct when an employee lowered the music volume, 音量が下げました or 音量を下げました?
Show the answer and why
音量を下げました is correct. The volume is the setting the employee adjusted, so it takes を. To state only the resulting decrease, say 音量が下がりました。