JLPT N2 Shinkanzen Grammar Lesson 1: ～とき・～直後に (When... / Immediately after...)

Meaning: ～際（に） means “when / on the occasion of...”. It is close to とき but is more formal and written in tone.
✓ It is used for a particular time or situation, especially in notices, instructions, procedures, business, and other formal contexts.
✓ It is not limited to intentional actions; it can also describe unintended events such as 火災が発生した際, “in the event of a fire”.
✓ With nouns: Nの際（に）; with verbs, Vる / Vた + 際（に） is common.

Meaning: ～に際して / ～にあたって can both mean “on the occasion of / when / in undertaking...”, and are commonly used for important events or occasions in a formal register.
✓ に際して highlights the occasion or situation being faced and has a fairly broad range, including unfavorable situations.
✓ にあたって often highlights embarking on, beginning, or undertaking something important, especially when preparation, resolve, or a formal statement is involved.
✓ Do not memorize absolute rules that they are “only for one-time events” or that にあたって is only for happy occasions; the distinction is mainly one of nuance and context.

Meaning: Vた + とたん（に） means “the moment... / just as...”. One event occurs immediately after the action or change in the first clause reaches its point of completion.
✓ The second event is typically unexpected, surprising, or outside the speaker’s deliberate control.
✓ Because the pattern commonly narrates an event that actually occurred, the second clause is very often past tense; however, “always past tense” should not be treated as an absolute formal rule.
✓ Commands, invitations, and deliberate intentions are generally unsuitable in the second clause.

Meaning: ～（か）と思うと / ～（か）と思ったら describes two states or events occurring in very quick succession: “just when it seemed... / no sooner... than...”.
✓ It often emphasizes a rapid, unexpected, or contrasting change.
✓ In this rapid-succession use, it normally describes phenomena or another person’s actions rather than a sequence of deliberate actions performed by the speaker.
✓ However, “never used for the speaker’s own actions” should not be treated as an absolute rule for every use of と思ったら, since the expression has other context-dependent uses.

Meaning: ～か～ないかのうちに means “hardly... when / almost as soon as...”, emphasizing that the second event occurs virtually at the same time the first event reaches completion.
✓ Typical forms are Vるか Vないかのうちに or Vたか Vないかのうちに, using the same verb in affirmative and negative forms.
✓ It highlights an extremely narrow time boundary, as if it is difficult to tell whether the first action has fully finished yet.
✓ It often occurs with actions or changes having a relatively clear starting or ending point, but V1 need not be memorized as exclusively a “momentary verb”.