かと思ったら and かと思うと mean "no sooner had… than": one thing happens, and straight after it comes something you didn't see coming. The verb before them is in the た-form.
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No sooner had the sky gone dark than it started to pour.
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No sooner had the baby fallen asleep than it woke up and started crying.
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He'd only just come home when he went out again.
Both halves are things you watch happen, so neither is your own action, and the second half is never a request or a plan.
- × 私は座ったかと思ったら、すぐ立ち上がった → 彼は座ったかと思ったら、すぐ立ち上がった
かと思ったら or かと思うと
Both carry "no sooner than". かと思うと is more written and suits a narrator describing a scene; かと思ったら is the one you hear in conversation. かと思ったら can also mean "I thought…, but actually": you assumed one thing and found another.
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No sooner had the lightning flashed than there was a loud bang.
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I thought my son was quietly studying, but he was playing games.
Usage notes
These go beyond かと思ったら and かと思うと themselves.
たとたん
also means "the moment that", and puts the weight on timing rather than surprise. It works for your own actions too.
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The moment I opened the door, the cat ran out.
かと思えば
sets two opposites side by side: one moment this, the next that.
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One moment she's crying, the next she's laughing.