N4
他動詞・自動詞
たどうし・じどうし
Done through action vs. occurring on its own
ローソクを消す。
To put out a candle.
Explanation
Many verbs come in pairs: transitive and intransitive.
1Transitive: someone does the action, and the object takes 「を」. 「（私が）窓を開ける」 (I open the window).
2Intransitive: something happens or is in a state by itself, and the subject takes 「が」. 「窓が開く」 (The window opens).
Common pairs: 「落とす・落ちる」, 「つける・つく」, 「見つける・見つかる」, 「下げる・下がる」, 「閉める・閉まる」, 「出す・出る」, 「消す・消える」.
Mixed up with
「窓が開いている」 just says the window is open; 「窓が開けてある」 means someone opened it on purpose, for example for fresh air.
In questions, look at the particle: 「を」 means a transitive verb; 「が」 usually means an intransitive verb, or 「transitive＋てある」.