In short

Japanese has a whole shelf of “the moment X happened, Y happened”: 〜が早いか (ga hayai ka), 〜や否や (ya ina ya), 〜なり (nari), 〜たとたん(に) (ta totan ni), 〜そばから (soba kara) and 〜たかと思うと (ta ka to omou to). They are narrative devices, not scheduling devices: the sentence almost always ends in the past tense and reports something that already happened. Learn to read all six; produce 〜たとたん first.

The rule

Step 1 - What the whole family shares

Every pattern here joins two clauses: [clause A] + PATTERN、[clause B in the past]。 B follows A with no perceptible gap, and the speaker is reporting, not planning. That single fact generates almost all the restrictions:

• B is past tense (〜た／〜ました). You are telling a story about what happened.
• B is not volitional, not a request, not a command, not an invitation. ×〜や否や、行きましょう ×〜が早いか、電話してください.
• Neither clause is negative. ×行かないが早いか.
• The subject is typically third person - you are watching someone else’s abrupt behaviour.
• Register is written or storytelling. These belong to novels, news and anecdotes, not to everyday chat.

〜たとたん is the partial exception on the last two points: it is common in ordinary speech, and it works fine with I as the subject, because what B reports is beyond your control (立ち上がったとたん、めまいがしました - you did not choose the dizziness). 〜そばから is the exception on the first point, for the reason in Step 4.

Step 2 - 〜が早いか and 〜や否や: the pure “no sooner than” pair

V-dictionary + が早いか literally asks “was A even the earlier of the two?” - the two events were so close you could not order them. Note the form carefully: the clause is dictionary form even though the event is in the past. 席に着くが早いか、食べ始めました (seki ni tsuku ga hayai ka) - “the moment he sat down, he started eating.”

V-dictionary + や or + や否や is the same meaning one notch more literary; 否や is the classical “or not”. Plain や is the shorter variant, や否や the formal full form you will meet in written Japanese. Both take the dictionary form too.

Neither adds any evaluation: they simply weld two events together. Use them when the speed itself is the point.

Step 3 - 〜なり: immediate and out of character

V-dictionary + なり adds something the other two do not have: the action in B is sudden, unexpected, or unlike the person. 家に帰るなり、部屋に閉じこもりました (ie ni kaeru nari) - he came home and shut himself in his room, which is exactly the kind of abrupt behaviour なり is for.

Two extra restrictions apply to なり alone:

1. なり keeps its focus on one actor, normally someone else. B continues with that same person, and it is not your own deliberate action: ×彼が帰るなり、私は寝ました. A different subject in B is tolerated only when B is something nobody controls - 家を出るなり、雨が降り出しました (ie o deru nari) is fine.
2. B is past tense - it is pure narration.

Step 4 - 〜たとたん(に) and 〜そばから: the two odd members

V-た + とたん(に) (often written 途端) means “the instant after”. It attaches to the た-form only, and B is normally something unexpected that the speaker did not intend. This is the accessible member: natural in speech, natural with a first-person subject, and the one you should actually produce.

V-dictionary / V-た + そばから is the only member that describes repetition, not a single event: B undoes A as fast as A is done, again and again. 片づけるそばから、子どもが散らかします (katazukeru soba kara) - “as fast as I tidy, the children mess it up.” Because it is habitual, そばから normally takes a non-past main clause - the one place in this lesson where 〜ます／〜る is the natural ending. A past main clause is possible when the repeated undoing is itself being told as past: ダイエットすると言ったそばから、ケーキを食べていました。

Step 5 - 〜たかと思うと／〜たかと思ったら: “I had barely…”

V-た + かと思うと (or かと思ったら) means “no sooner had A happened than B” with an audible note of surprise - the speaker had hardly registered A before B overtook it. Because the surprise is the speaker’s, this pattern is not used about your own deliberate actions: ×私は座ったかと思うと、立ちました.

Step 6 - Do not put 〜次第 in this family

V-ます-stem + 次第 (lesson 64) translates as “as soon as” too, but it is the mirror image of everything above: it is scheduling, it points at the future, and it therefore requires a non-past main clause and happily takes a volitional, a request or a promise - 準備ができ次第、出発します。That is the practical test: if you can still choose whether B happens, you need 次第; if B is already history, you need this lesson.

Forms & patterns

Attachment and meaning:

The shared restrictions - and who breaks them:

Examples

Common mistakes

• 家に着いたが早いか、電話しました。 (Ie ni tsuita ga hayai ka, denwa shimashita.) -> 家に着くが早いか、電話しました。 (Ie ni tsuku ga hayai ka, denwa shimashita.) - が早いか and や否や take the dictionary form, however clearly past the event is. Only the main clause carries the tense.
• ドアを開けるとたん、猫が飛び出しました。 (Doa o akeru totan, neko ga tobidashimashita.) -> ドアを開けたとたん、猫が飛び出しました。 (Doa o aketa totan, neko ga tobidashimashita.) - とたん is built on the た-form only. It is the mirror image of が早いか, so keep the pair straight.
• 駅に着くや否や、電話してください。 (Eki ni tsuku ya ina ya, denwa shite kudasai.) -> 駅に着いたら、電話してください。 (Eki ni tsuitara, denwa shite kudasai.) - these patterns report finished events, so a request or command cannot follow. For instructions use 〜たら or 〜次第.
• 彼が帰るなり、私は寝ました。 (Kare ga kaeru nari, watashi wa nemashita.) -> 彼は帰るなり、寝てしまいました。 (Kare wa kaeru nari, nete shimaimashita.) - なり stays on one actor - B is that person’s next move, never the speaker’s own deliberate action. With two different actors, use 〜と or 〜たとたん.
• 覚えるそばから、復習しました。 (Oboeru soba kara, fukushū shimashita.) -> 覚えるそばから、忘れてしまいます。 (Oboeru soba kara, wasurete shimaimasu.) - そばから needs B to cancel A, repeatedly and habitually - 復習する does not undo 覚える. A single past event is 〜たとたん.

Tips

• One form fact separates the two you will actually meet. が早いか takes 辞書形, とたん takes た形. Say 着くが早いか／着いたとたん aloud together until the pairing is automatic; getting it backwards is the classic C1 slip.
• English “as soon as” splits into two Japanese halves. “As soon as it rang, they left” is narrative -> this lesson. “As soon as it is ready, I will send it” is a plan -> 〜次第 or 〜たら. English hides the difference; Japanese does not let you.
• そばから is the only plural one. Everything else here happens once. If your English would be “no matter how often I …”, そばから is the pattern; if it is “the instant that …”, it is not.
• Read them, then produce one. が早いか, や否や and なり are reading vocabulary for a long time - a misplaced なり is very conspicuous. In your own Japanese, 〜たとたん covers most of the ground safely, and 〜たら covers the rest.