～かと思うと／かと思ったら— No Sooner Than, Just When

• 赤ちゃんは泣いたかと思うと、すぐに笑い出した。

  Akachan wa naita ka to omou to, sugu ni waraidashita.

  No sooner had the baby cried than it burst out laughing.

  泣いた (た-form) + かと思うと marks the instant switch to the next action.

• 空が暗くなったかと思ったら、大雨が降ってきた。

  Sora ga kuraku natta ka to omottara, ōame ga futte kita.

  Just when the sky darkened, a heavy rain began to fall.

  暗くなった (た-form) + かと思ったら introduces the immediately following event.

• 彼は帰ってきたかと思うと、また出かけて行った。

  Kare wa kaette kita ka to omou to, mata dekakete itta.

  No sooner had he come home than he went out again.

  帰ってきた (た-form) + かと思うと shows the surprising quick reversal.

• ・The second clause is a past/spontaneous event, not the speaker's own planned action, command, or request.
• ・Conveys surprise at an abrupt change; often two contrasting events.
• ・Register: neutral to slightly literary; common in narration.
• ・The subject is usually a third party or a natural phenomenon.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～かと思うと／かと思ったら is tested against.

• に際して — On the Occasion Of
• に先立って — Ahead of the Event
• につけ — At Every Turn
• ～にわたって — Spanning the Whole Range
• ～たび(に) — Every Single Time
• ～あげく(に) — After All That, In the End
• ～折(に) — On the Occasion Of
• ～末(に)／の末(に) — After All Was Said and Done