Though "as" itself is hard enough in Japanese, Japanese also has a lot of expressions that mean “as soon as”. This lesson will investigate how to use these expressions. Take close attention to what defines them as there are differences!
Placed after the 終止形 of a verb in ～や or ～や否や, it means "as soon as". With 否や, it is like the second action takes place before the first action can even be confirmed or done. The most used is や否や. In fact, や否や will be used 90% of the time.
As soon as the earthquake happened, the Meteorological Agency issued a tsunami warning.
I began weeping as soon as I saw my sister's face.
3b. その選手はチャンスと見るやいなや、一気に｛攻め込んだ・攻撃をしかけた｝。 (あまり使わない言い方)
The player felt a chance open and attacked in one burst.
It started to rain as soon as I departed.
～やいなや cannot be used to show the speaker's wants or intentions. Also, don't use the past tense before ～やいなや. The action can be something that you can expect. It’s important to understand that the events of the second clause are actions/movements.
My cat loves fish, and the instant you give it to it, it's gone in an instant.
After the 連体形 of a verb, ～なり shows that something is done as soon as something else is done. So right when someone does something, they do something next in sequence to the first action. The subject is normally third person, and the subject is the same in both clauses.
6b. 宿題を済ませるとすぐに、彼らはインターネットを使った。(More natural)
They used the Internet as soon as they finished their homework.
He went to the bathroom as soon as he got home.
As soon as the company president came in, he shouted in a big voice.
The word comes from the なり in words like 身なり (appearance). The event in the second clause is often one that describes an action/condition not wanted. Although it is used some in the spoken language, it is usually reserved to writing.
After the past tense, it shows a situation that is still in play as another action begins. It is unnatural when you move in any way to a different action.
9. 彼は靴を履いたなり、畳に上がってしまった。 △
He accidentally stepped onto the tatami mat with his shoes still.
She fell asleep while sitting on a park bench.
When students learn of 途端に, they immediately think of ～ときに, especially when they learn that this phrase is used after the past tense. Understandable, ～たとき（に）and ～たとたん（に） vaguely resemble each other, especially when not written in 漢字.
～た途端（に） is, again, only used with the past tense. It marks the instant after one does something. This “something” that happens afterward is something that is unexpected and much of a surprise.
The car exploded just as he opened the door.
As soon as I opened the window, the dog jumped out.
As soon as he left, the building exploded!
I accidentally fell asleep as soon as I saw the video.
I fell down as soon as I turned away.
Once a person drinks, that person changes instantly.
Grammar Note: 途端 may also be used in the sense of instantly and can be seen with such statements.
As seen in the example sentences, though, ～た途端（に）can’t be used when something volitional occurs. So, although a situation may be unexpected, you would need to use a phrase like ～たら、すぐに if you are using a verb of volition.
When the bell rings, immediately go outside.
Perhaps because it is rarely treated separately in grammar discussions, students have a hard time knowing how to use this phrase. This describes something happening that causes a sense of surprise, similar to ～た途端（に）, which follows after an event described in the first clause. Transitivity, though, is clearly different as ～た途端（に）is used with transitive expressions, but this phrase does not have that requirement.
You cannot use this phrase for yourself. At times it may be best to use ～かと思ったら instead. Sentences of command, negation, or will appear afterwords. For some people, when the event is not of the future or going back in forth in a way that means the first action may happen again in the "future", then the use of ～かと思うと is somewhat unnatural and should be replaced with ～かと思ったら. So, for the sentences below regarding weather, you can paraphrase with the latter and have no problem.
18. 雨が降ってきたかと｛思うと ?/〇・思ったら 〇｝、もう止んだ。
Just when I thought it had started to rain, it stopped.
19. 空が曇ってきたと｛思うと ?/〇・思ったら 〇｝、突然大雨になった。
Just when I thought it got cloudy, it started to rain heavily.
Just as I had thought she said she liked A-kun, she then stated that she liked B-kun, and so I don't know what she's thinking.
22. うちの子供は帰ってきたかと｛思うと ?/〇・思ったら 〇｝、もう外に遊びに行った。
The kids had already gone to play outside just when I thought they had come home.
As a regular noun, 次第 means "course of events". It is seen after nouns a lot, especially Sino-Japanese words, but also after the 連用形 of verbs.
When it attaches to other nouns, it often means "depending on". As such, when after the 連用形 of a verb, it may show that something is dependent on an action in question.
It's on demand.
Circumstances alter cases.
This is how it stands.
It depends on the way you handle it.
I'm going to decide on whether to go or not depending on the weather condition.
The president's resolution depends on this.
An abundant harvest is dependent on the weather.
The results depend on you.
Promotion is dependent on one's ability.
連用形 + 次第
It may also be used with the 連用形 of a verb to show what happens "as soon as...". This is in the sense that right after something realizes, one does the next action. Thus, the second clause following must be a verb of volition in regards to the speaker. This separates it quite well from the other “as soon as” phrases in this lesson.
As soon as we arrive, we will aid in the disaster area.
To buy up food as it is made available.
Let's close up as soon as we become a full house.
We plan to resume operations as soon as it stops snowing.
Lastly, 次第に is an adverb meaning "gradually", "finally", or "in order".
Machines will gradually go out of date.
Noise will gradually fade away.