なり

Expresses that immediately after one action, the same person does a second, often sudden or unexpected action. Equivalent to “as soon as,” “the moment (that),” or “no sooner had … than.”

A somewhat literary pattern, typical of narration and written descriptions of someone's behavior. Both clauses usually share the same subject, typically a third person, and the second clause describes a completed, often abrupt reaction. It differs from 次第, which introduces a planned future action, and from なり〜なり, which lists alternatives.

• や否やN1「や否や」 is close to 「なり」 in how it connects and in register, but its main clause isn't limited to unexpected actions: it often describes situations in which the two clauses have different subjects, and it can be used in first-person narration. When describing someone else doing something and abruptly acting right after, the two are interchangeable.
• が早いかN1Besides the dictionary form, 「が早いか」 can also attach to the た-form, and it stresses speed so great that the second action almost jumps ahead of the first; its main clause must state something that actually happened, not a conjecture. For events that have already happened, the two are mostly interchangeable, and both are rather literary.
• たとたんN3「たとたん」 attaches to the た-form, is neutral in register, and is more common; its main clause usually describes an uncontrollable change or someone else's action, and it can also relate the speaker's own experiences. When describing a third person finishing something and an action following at once, the two can often be swapped.
• そばからN1「そばから」 emphasizes the same thing happening over and over so that effort is wasted, usually with a sense of resignation or annoyance, and is not used for one-off events; it attaches to the dictionary form or the た-form and works in both speech and writing. Since 「なり」 focuses on an unexpected main clause, the two are generally not interchangeable.
• なり〜なりN1「なり〜なり」 is used in pairs after verbs or nouns to mean "either … or", offering a choice of one, and is usually followed by a suggestion or command; 「なり」 on its own follows a verb's dictionary form to mean "as soon as", and its main clause cannot be a command. They are not interchangeable.
• 次第N2In its "as soon as" sense, 「次第」 attaches to the ます-stem (with ます dropped) or to an action noun, refers to future events, and usually takes a main clause expressing intention or a request, while 「なり」 attaches to the dictionary form and narrates past events, so they cannot be swapped. 「次第」 also has a "depends on" meaning.

Like や否や and が早いか, なり means “as soon as,” but those two are more literary, and が早いか stresses the speed of the reaction. たとたん is more conversational and often describes an event beyond anyone's control, while なり focuses on the same person's abrupt next action. そばから describes an action immediately undone, often repeatedly; なり〜なり lists alternatives; 次第 introduces a planned future action.