〜や否や（文語）

〜や否や is a formal expression (文語（ぶんご）) used to link two actions where the second follows the first without any delay. The character 否（いな） means 'no' in classical Japanese. Literally translating to 'doing [Action] or not,' it implies the second action happens so fast that it is indistinguishable from whether the first action finished or not. It also has a secondary meaning of 'whether or not' (synonymous with かどうか).

Key Characteristics:

• Register: It is highly formal and is almost never used in casual conversation. Using it in daily speech would sound overly dramatic or like a narrator in a story. A shortened variant, 〜や, is also commonly used in literature with the same meaning.
• Momentary Action: The first verb must be a momentary action (e.g., arriving, seeing, opening) rather than a continuous state. It cannot attach to stative verbs (状態動詞（じょうたいどうし）) like いる, ある, or できる. Furthermore, the second clause must also express an active movement or sudden event, not a continuous state.
• Subject Flexibility: The subject of the first clause and the second clause can be different (e.g., 'As soon as the bell rang, the students left'). The subject is usually in the third person. However, it is perfectly natural and standard to use a first-person subject if the second clause describes a non-volitional, spontaneous reaction (e.g., 薬を飲むや否や、急に眠気がおそってきた).
• Event Relationship: The two events are typically closely related, implying a direct, anticipated reaction or a sequence where the second action seems to have been waiting for the first. Action A typically triggers Action B, creating a causal relationship. However, it can also be used for sudden natural occurrences or coincidences, where it adds a dramatic, personified nuance.
• Immediacy and Surprise: It emphasizes the extreme immediacy of the sequence. It frequently carries a strong nuance of surprise or unexpectedness at the sheer speed of the second event, and is used for both anticipated actions and completely unexpected reactions.
• Grammar Rule: It attaches strictly to the Dictionary Form of a verb. Unlike some other time-related grammar points (such as 〜際に), it cannot attach to nouns or adjectives. It is used to describe events that have already happened, so the main clause almost always takes the past tense.
• Restrictions: It cannot be used with the speaker's will, commands, requests, or invitations (e.g., 'As soon as you get home, please call me' is incorrect with this pattern). First-person volitional actions (e.g., 'I got home and immediately started studying') are also unnatural. However, a third party's volitional action is perfectly acceptable and common. Additionally, the second clause cannot be a negative sentence or a continuous state.
• Comparison with 〜なり: Unlike 〜なり, which typically requires the same third-person subject for human actions (though it can take different subjects for non-human entities), 〜や否や freely allows the first and second clauses to have different subjects and permits first-person subjects when the second clause is a non-volitional reaction.