To become open (e.g. a door or window opens); an intransitive action of opening.
ドアが開く音がして部屋に誰か入ってきた。
There was the sound of a door opening and someone entered the room.
窓を開くと涼しい風が入ってきた。
When I opened the window, a cool breeze came in.
夕方になると店のシャッターが開く。
In the evening the store's shutter opens.
To open for business or start operating (e.g. a shop or event opens).
新しいカフェが来週から開くらしい。
It seems a new cafe will open from next week.
夏祭りは午後五時に開く予定だ。
The summer festival is scheduled to open at 5 PM.
会社は月曜日の朝9時に開く。
The company opens at 9 AM on Monday.
To become empty or unoccupied (e.g. a seat or space becomes empty).
電車の座席が空くまで立って待った。
I stood and waited until a seat on the train became available.
ゴミ箱が空いているかどうか確認して。
Check whether the trash can is empty.
昼休みのあとでは会議室が空いていないことが多い。
After lunch, the meeting rooms are often not available.
To be available or vacant (e.g. time slot, seat, or appointment is free).
明日の午後は予定が空いていますか？
Are you free tomorrow afternoon?
週末に予約を取れる席はまだ空いている。
There are still seats available for booking on the weekend.
来週なら時間が空くので会いましょう。
I'll be free next week, so let's meet then.
To be open (e.g. a neckline or collar is open).
彼女のワンピースは胸元が大きく開いていた。
Her dress had a low/open neckline.
コートのボタンが一つ開いているよ。
One of your coat buttons is undone.
To have one's eyes or mouth open (e.g. open wide), often used for reacting or waking up.
子供はびっくりして目を開いた。
The child opened his eyes in surprise.
彼女は眠りから覚めて口を開いたままだった。
She woke up and remained with her mouth open.
To come to an end or transition (e.g. night ending and dawn coming).
長い議論が終わり、ようやく時が開いたように感じた。
The long discussion ended, and it felt as if a new time had begun.
夜が明くとともに、町に人影が戻ってきた。
With the dawn, people began returning to the town.
Used transitively to open something such as one's eyes or mouth (to open one’s eyes/mouth).
彼は眠気に耐えられず、無理に目を開いた。
He couldn't stand the sleepiness and forced his eyes open.
驚いて思わず口を開いてしまった。
Startled, I unintentionally opened my mouth.
To have a hole or gap form (e.g. a hole appears in something; an interval opens between events).
古い靴に穴が開いてしまった。
A hole has formed in my old shoes.
二つのイベントの間に時間の空きが開いた。
A gap of time opened up between the two events.