Verb Transitivity

In Japanese, there are some verbs that come in pairs to indicate transitivity. For example, 開ける versus 開く, which mean "to open something" and "something opens" respectively.

カナトはドアを開けた。
Kanato opened the door.

ドアが開いた。
The door opened.

Transitive verbs tend to describe activities, while intransitive verbs tend to describe changes. This is especially important to remember when using the progressive ている conjugation. For transitive verbs, this usually means that an action is in progress, while for intransitive verbs, it usually describes a resultant state.

アヤトは窓を開けている。
Ayato is opening the windows.

窓が開いている。
The windows are open.

Here are some other common transitivity pairs. There are many more, but these are some of the more useful ones.

閉める　  閉まる     -  close something vs. something closes
入れる　  入る        -  put something in vs. something enters
出す　      出る        -  take something out vs. something goes out
つける　   つく         -  turn something on vs. something turns on
消す　      消える    -  turn something off vs. something goes off
壊す　      壊れる    -  break something vs. something breaks
汚す　      汚れる    -  make something dirty vs. something becomes dirty
落とす　   落ちる    -  drop something vs. something drops
沸かす　  沸く         -  boil water vs. water boils