1For something like a door or window to move from a closed state to an open one.
風でドアが勝手に開いた。
The door opened by itself because of the wind.
このふたはなかなか開かない。
This lid won't open easily.
エレベーターのドアが開いたら、すぐに乗ってください。
As soon as the elevator door opens, please get on right away.
玄関の鍵をかけたはずなのに、なぜか開いていた。
I was sure I'd locked the front door, but somehow it was open.
2For a shop, facility, or event to begin operating or open for business.
そのパン屋は朝7時に開く。
That bakery opens at 7 a.m.
新しい図書館が来月開くらしい。
It seems a new library will open next month.
展覧会は10時に開くので、それまで待とう。
The exhibition opens at 10, so let's wait until then.
3For a seat or space to become empty or unoccupied.
バスの前の席が空いたので座った。
A seat at the front of the bus became free, so I sat down.
冷蔵庫の中がすっかり空いている。
The inside of the refrigerator is completely empty.
一人が引っ越して、部屋が一つ空いた。
One person moved out, so one room became vacant.
4For a schedule, time slot, or seat to be free or available.
来週の水曜なら時間が空いています。
I'm free next Wednesday.
そのレストランは今、席が空いていますか。
Is there a table available at that restaurant right now?
予定が空いたら、また連絡します。
If my schedule opens up, I'll contact you again.
5For a garment's neckline or collar to be open wide.
そのドレスは背中が大きく開いていた。
That dress had a low-cut, open back.
シャツのボタンを一つ外したら、少し胸元が開いた。
When I undid one shirt button, the neckline opened up a little.
6For one's eyes or mouth to become open (e.g. wide open), often used for surprise or waking up.
赤ちゃんが眠りながら少し口が開いていた。
The baby's mouth was slightly open while sleeping.
手術後、しばらく目が開かなかった。
After the surgery, my eyes wouldn't open for a while.
7For a period of time to come to an end and transition into the next state (e.g., night ending into dawn).
夜が明くころ、鳥の声が聞こえ始めた。
As the night came to an end, birdsong began to be heard.
喪が明くまで、しばらく静かに過ごした。
We spent some time quietly until the mourning period ended.
8The transitive use meaning to open something like one's eyes or mouth (by one's own action).
医者に言われて、目を大きく開いた。
As the doctor told me, I opened my eyes wide.
歯医者で口を大きく開いてくださいと言われた。
At the dentist, I was told to open my mouth wide.
9For a hole or gap to form in something, or for an interval to open up between events.
靴の底に穴が開いてしまった。
A hole has formed in the sole of my shoe.
二つの会議の間に一時間ほど間が開いている。
There's about an hour's gap between the two meetings.