Hardly… when: 〜が早いか

A が早いか B says B happens almost at the same time as A, so fast you can't tell which came first. It's used in a narrative, in the past, for a quick and often surprising action. No plans or requests in the second part. Written, narrative register.

• 家に着くが早いか、彼は寝てしまった。いえにつくがはやいか、かれはねてしまった。He'd hardly got home when he fell asleep.
• 子どもたちは、おやつを受け取るが早いか、外へ駆け出していった。こどもたちは、おやつをうけとるがはやいか、そとへかけだしていった。The children had barely got their snack when they dashed outside.
• 彼は給料をもらったが早いか、全部使ってしまった。かれはきゅうりょうをもらったがはやいか、ぜんぶつかってしまった。He'd hardly been paid before he'd spent the lot.

Literally: “was A quicker?”. You can no longer tell whether A came before B, B followed so fast. The formula stresses haste, often with a touch of astonishment or amusement.

• 家に着くが早いか、寝てしまった。いえにつくがはやいか、ねてしまった。He'd hardly got home when he fell asleep.
• 信号が青に変わるが早いか、車は一斉に走り出した。しんごうがあおにかわるがはやいか、くるまはいっせいにはしりだした。The lights had barely turned green when all the cars shot off together.

が早いか goes after the dictionary form or the た-form. Both are possible; the た-form stresses that A is completed. The sentence ends in the past.

• 彼女は荷物を置いたが早いか、また出かけていった。かのじょはにもつをおいたがはやいか、またでかけていった。She'd barely put down her bags when she'd gone out again.

が早いか is for storytelling. It isn't used to announce what you're going to do (“着くが早いか電話します” is wrong) or to give an order. It's also rare to apply it to your own intentions: you tend to describe a third party, seen from outside.

• 犯人は警官の姿を見るが早いか、路地に逃げ込んだ。はんにんはけいかんのすがたをみるがはやいか、ろじににげこんだ。The criminal had barely spotted the police officer when he dived into an alleyway.

All four translate as “as soon as, hardly”. が早いか is the most focused on speed, near-simultaneity. や否や is very close and a little more frequent in the press. なり requires the same subject and an unexpected reaction. とたん is the everyday version, often for something unexpected.

• 窓を開けたとたん、強い風が吹き込んできた。まどをあけたとたん、つよいかぜがふきこんできた。The moment I opened the window, a strong wind rushed in.