かと思ったら / かと思うと|かとおもったら / かとおもうと

Expresses that a second event happened immediately after the first, often unexpectedly or in contrast to it: 'no sooner had … than …', 'just when (I noticed) …, …'. The speaker describes a sudden change they observed.

Used in both speech and writing to narrate a quick, surprising change the speaker witnessed. The two events often contrast, as in starting and stopping or crying and laughing, and the second clause frequently contains すぐ or もう. たとたん simply marks that the second event occurred at the very moment of the first, whereas this pattern foregrounds the speaker's surprise at noticing how fast things changed.

• と思いきやN1「と思いきや」only expresses 'I thought... but to my surprise...', which is close to this pattern's 'I had thought' sense, so the two are often interchangeable; however, it has a more literary feel and cannot express the back-to-back 'no sooner... than...' contrast.
• たとたんN3「たとたん」attaches only to the ta-form of verbs and focuses on two events happening almost simultaneously, with no necessary contrast; it can describe the speaker's own actions but lacks the 'I thought... yet...' meaning. When describing other people or things changing suddenly, the two are often interchangeable.
• なりN1「なり」attaches to the dictionary form, is more literary, and only means 'no sooner... than...', typically describing an unexpected action immediately following another, without stressing contrast; 「かと思ったら」in this sense takes the ta-form, is stylistically neutral, highlights the contrast, and additionally carries the 'I had thought' meaning.
• そばからN1「そばから」is used only for things that happen repeatedly, often implying that the effort of the first action is undone by the second, with a nuance of resignation and annoyance; it attaches to either the dictionary form or the ta-form, the first action can be the speaker's own, it has no 'I had thought' meaning, and the two are generally not interchangeable.
• が早いかN1「が早いか」covers only the 'no sooner... than...' sense and has no 'I had thought' meaning; it attaches to either the dictionary form or the ta-form, stresses actions so quick they are nearly simultaneous rather than any contrast, and appears mostly in written narration such as novels, rarely in everyday conversation.
• や否やN1「や否や」corresponds only to the 'no sooner... than...' sense of 「かと思ったら」, attaches to the dictionary form of verbs, is strongly literary, emphasizes only the extremely short interval without requiring any contrast, and can be used in the first person; it has no 'I had thought' meaning.

と思いきや marks an expectation that turned out wrong. かと思ったら / かと思うと narrates a rapid, often contrasting change the speaker observed, while たとたん marks that something, often unexpected, happened at the very moment of the first event, without implying that the first event caused it. なり, が早いか, and や否や are more literary forms that focus on someone acting the moment something happens, and そばから describes an effort being undone repeatedly as soon as it is made.