〜やいなや

As soon as; the moment (that). It describes a second event happening immediately after the first, with almost no gap in time.

A literary expression used mainly in written narratives, novels, and news to describe actual events. The second clause often reports a sudden or striking reaction, and the two clauses may have different subjects (example 2). It is stiffer than たとたん, which is the everyday equivalent.

• 1.
  なりN1なり also attaches to the verb dictionary form to mean 'as soon as', but the second action is usually an unexpected one, the subject is generally the same in both clauses, and it is generally not used for the speaker's own actions. When describing another person's sudden action, the two patterns are often interchangeable.
• 2.
  が早いかN1が早いか has almost the same meaning as や否や, and the two are often interchangeable. However, が早いか can also attach to the た-form in addition to the dictionary form, as in 「言ったが早いか」, emphasizing speed to the point of almost preempting the first action, and it is generally not used for actions the speaker deliberately arranges.
• 3.
  そばからN1そばから also means 'as soon as', but it emphasizes the same thing happening over and over and rendering the first action's efforts futile, often with a nuance of helplessness or frustration, so it cannot be used for one-time events. It attaches to either the dictionary form or the た-form and works in both writing and speech.
• 4.
  たとたんN3たとたん is close in meaning but attaches to the verb た-form (「入ったとたん」); its register is neutral, it is more colloquial and more frequently used, and the second clause often carries a sense of suddenness or surprise. When narrating things that have already happened, the two are often interchangeable, and たとたん is the better choice in everyday conversation.
• 5.
  た拍子にN1た拍子に attaches to the verb た-form, has a neutral register, and focuses on how the impact of the first clause's instantaneous action directly triggers an unexpected, non-volitional result. や否や only states temporal immediacy and allows a deliberate action in the second clause, so such sentences cannot be rewritten with た拍子に.

Unlike なり, which requires the same subject in both clauses, や否や allows different subjects. が早いか is similarly literary but stresses near-simultaneity. そばから describes repeated situations in which a result is undone right away. たとたん is the more common, conversational equivalent and attaches to the た form. た拍子に marks an accidental result triggered by the first action.