“〜ittan 〜tara/to”: Once 〜, Then 〜 (hard to turn back once begun) [JLPT N2]

Content by NTJ Kuma · Nihongo to Japan editorial team · Updated July 27, 2026

“Once you start, see it through,” “once decided, no wavering” — once begun, hard to turn back

「いったん決めたら、最後までやる」 — once you cross the line, there's no turning back

「いったん引き受けたら、途中で投げ出すわけにはいかない」 (once you take it on, you can't just abandon it midway) — 「いったん〜たら／と」 expresses “once ～, (then) ～”: once something holds, once it's done, a result inevitably follows, or is hard to reverse. This guide covers 「いったん」's use and attachment, separating it from the plain 「一度〜たら」 (N2).

Core: once ～ (then) ～ (once it holds, hard to change/inevitable)

「いったん ＋ (verb た-form) ＋ たら／(verb dictionary form) ＋ と」 = “once ～, (then) ～” — stressing “once you cross a certain point (done it, it holds), what follows inevitably occurs or is hard to turn back from”:

• いったん約束したら、必ず守るべきだ (once you promise, you must keep it).
• パソコンはいったん故障すると、修理に時間がかかる (once a computer breaks, repairs take time).

Feel: “once ～, (then) ～” — the core is “once something holds, it enters a state of ‘can't easily turn back' or ‘inevitably so.'” 「いったん」 itself means “once, for the time being”; paired with 「たら／と／ば」 it marks “once … (then …),” with a “cross a threshold, hard to reverse” feel.

Attachment & forms

Examples in use

• いったん火がつくと、あっという間に燃え広がる。(Once it catches fire, it spreads in a flash.)
• いったん引き受けた以上、責任を持ってやる。(Once I've taken it on, I'll do it responsibly.)
• この山は、いったん登り始めたら、途中で引き返せない。(Once you start climbing this mountain, you can't turn back midway.)
• 悪い噂は、いったん広まると、消すのが難しい。(A bad rumor, once it spreads, is hard to erase.)
• いったん**、休憩しましょう。(Let's take a break for now.) — いったん alone (for the time being)

⚠️ Common mistakes to avoid

① 「いったん〜たら／と」 carries a ‘hard to turn back/inevitable' feel, not plain sequence. It stresses “once you cross a point, what follows inevitably occurs or is irreversible” (いったん決めたら変えない = once decided, won't change). So the second clause is often “a hard-to-reverse, inevitable result” (can't abandon midway, hard to erase, will surely …). If it's just “after A, do B” (no irreversibility), the plain 「〜たら／たあとで」 suffices. いったん carries a “threshold, irreversible” flavor.

② 「いったん」 has two uses: ‘once … then' and ‘for now, first.' ① いったん〜たら／と = once ～ then (hard to turn back) (いったん始めると止まらない); ② いったん (alone) = for the time being, do once first (いったん家に帰る = go home first; いったん保留にする = put on hold for now). The latter just means “first, temporarily (do once),” without irreversibility. Check whether it's “paired with たら／と for once…then” or “alone for the time being.”

③ Close to 「一度〜たら」, but 「いったん」 stresses ‘an irreversible threshold' more. Both can mean “once … then,” but 「一度〜たら」 leans ‘after (having) done once' (= after one experience/action) (一度食べたら、やみつきになる = once you eat it, you're hooked); 「いったん〜たら」 stresses ‘once you cross, hard to turn back, entering a state' (いったん決めたら覆さない = once decided, won't overturn). To stress “cross a threshold, irreversible” use いったん, for “after one experience” use 一度. (Often interchangeable, with slightly different feel.)

🔄 Compare: いったん〜たら／と／一度〜たら／〜たら最後／いったん (alone)

💡 Learning tip

「いったん〜たら／と」 = once ～ (then) ～, its core “once you cross a certain point (done it, it holds), what follows inevitably occurs or is hard to turn back from” (いったん決めたら最後までやる; いったん広まると消せない). Three points: ① it carries an ‘irreversible, inevitable' feel, not plain sequence (the second clause is often a hard-to-reverse/inevitable result); ② 「いったん」 has two uses — with たら／と = “once … then,” alone = “for the time being, first (do once)”; ③ close to 「一度〜たら」 but stressing ‘threshold, irreversible' more, lighter than 「〜たら最後 (once …, doomed)」. To express the resolve or inevitability of “once you cross, no turning back,” いったん〜たら／と nails it.

❓ Questions you might have

Q: 「いったん」 and 「一度」 both mean ‘once' — how do I tell them apart?

Close, often interchangeable, but with different focus. 「いったん〜たら／と」 = once ～ (then) ～ (stressing ‘after crossing a threshold, entering a state, hard to turn back or inevitable'): いったん人を信じたら、最後まで信じる (once I trust someone, I trust them to the end — stressing “after crossing the ‘trust' threshold, the attitude doesn't change”); 機械はいったん止まると、再起動に時間がかかる (once the machine stops, restarting takes time — the inevitability after crossing the “stop” point). Its point is “once it holds, entering a hard-to-reverse state,” with a “threshold, irreversible” flavor. 「一度〜たら」 = after (having) once ～ (stressing ‘after one such experience/action,' then a result): 一度この味を知ったら、忘れられない (once you've tasted this, you can't forget it — a result after “one experience”); 一度失敗したら、二度と挑戦しない人もいる (some, after failing once, never try again). Its point is “after experiencing once (what happens).” Test: stress “cross a threshold, irreversible” → いったん; stress “after one experience/action” → 一度. Often interchangeable (いったん／一度 始めたら止まらない), but いったん's “irreversible, no turning back” flavor is stronger, 一度 leans “one experience.”

Q: 「いったん〜たら」 and 「〜たら最後」 are both ‘once ～ then' — same tone?

No — 「〜たら最後／〜が最後」 is the strongest tone and almost always takes a negative result. 「いったん〜たら／と」 = once ～ then (neutral-to-strong, can be positive or negative, stressing hard-to-reverse or inevitable): いったん決めたら、やり遂げる (once decided, I'll see it through — positive resolve); いったん広まると、消せない (once it spreads, it can't be erased — negative inevitability). It can take a positive (resolve) or negative (trouble) result, with a “once …, (hard to change)” tone. 「〜たら最後／〜が最後」 = once ～, that's it (doomed) (strongest tone, ‘once …, irrecoverable, dire consequences,' almost always negative): あの人に見つかったら最後、逃げられない (once that person finds you, that's it — no escape, dire); この秘密を知ったが最後、命が危ない (once you know this secret, that's it — your life's in danger). It stresses “once …, no recovery / a terrible fate,” most dramatic and negative. Test: neutral-to-strong, positive or negative “once …, (hard to turn back)” → いったん〜たら; strongest, negative “once …, doomed, irrecoverable” → 〜たら最後. For resolve (once decided, see it through) use いったん; to stress the dire “once …, no return” use たら最後.

🖊️ Practice Questions (5)

Q1. 「___始めたら、途中でやめられない。」(once you start, you can't stop)

(A) いったん　(B) いよいよ　(C) いまにも　(D) いかにも

Q2. 「いったん〜たら」の後件として典型的なのは?

(A) Hard to turn back / an inevitable result　(B) Purely temporal　(C) A question　(D) 命令の禁止のみ

Q3. 「いったん決めたら最後までやる」の意味は?

(A) Once you decide, you see it through to the end

(B) Haven't decided yet

(C) Often change your decision

(D) Don't make decisions

Q4. 「いったん」を単独で使うと?

(A) For the time being (いったん帰る)

(B) Forever

(C) Absolutely not

(D) Right away

Q5. 用法が自然なのは?

(A) いったん約束したら守るべきだ。　(B) いったん（理由は3つ）。

(C) いったんを食べる。　(D) いったんだ高い。

Answer Explanations

1. (A) いったん ── Once you start, you can't stop.

2. (A) ── The main clause is usually something hard to reverse / an inevitable result.

3. (A) ── Once you decide, you see it through to the end.

4. (A) ── On its own it means "for the time being".

5. (A) ── 「いったん約束したら守るべき」is correct.