This means that right after one thing happened, another thing happened very quickly. It shows a fast and often surprising change.
Verb [た-form] + かと思うとPrimary
Verb [た-form] + かと思ったら
This means that right after one thing happened, another thing happened very quickly. It shows a fast and often surprising change.
さっきまで晴れていたかと思うと、急に黒い雲が広がってきた。
Just when I thought it had been sunny until a moment ago, dark clouds suddenly spread across the sky.
あの選手はゴールを決めたかと思うと、もう次のプレーのために走り出していた。
No sooner had that player scored a goal than he was already running for the next play.
赤ちゃんはさっきまで笑っていたかと思うと、もう泣いている。
Just when I thought the baby was laughing a moment ago, it's already crying now.
新しい店ができたかと思うと、一年ももたずに閉店してしまうことが最近は多い。
Just when you think a new store has opened, they often close down without even lasting a year these days.
彼は部屋に入ってきたかと思ったら、何も言わずにすぐ出て行った。
Just when I thought he had come into the room, he left right away without saying anything.
This phrase shows surprise. Things changed very fast. You did not expect the second thing to happen. Think of a quick magic trick. It is often used for things that change quickly. Like the weather or a baby's mood.
This grammar shows two things happening fast. It is different from "~to tan (ni)". That one is for one surprising event. This grammar shows a quick change between two things. You cannot use it for your own actions. It is also different from "~ta bakari da". That one means something happened a short time ago. The actual time might be long.
Don't use this when you talk about your own choices. For example, don't say "I thought I came home, then left." This is for things you just see happening.
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