～となったら – If it comes to… / In the event that…

Preceding element	Structure with ～となったら	Core meaning	Form notes
Noun (N)	N + となったら	If/when it comes to N; once it becomes N...	Usually marks a time point/event: 本番、来週、雨、中止、引っ越し
Verb	V-辞書形 + となったら	If/when it is decided/comes to doing V...	Often used with decisive actions: 結婚する、留学する、引っ越す
Adjectives -na/-i	A + となったら	If/when it becomes A...	Seldom used with pure adjectives; sounds more natural with N/V

Core meaning: “If/when (something) becomes/is decided as A, then B.” Emphasizes the moment a state changes or a decision is set.
Nuance: leans toward the speaker’s standpoint, often followed by the speaker’s intention/suggestion/planned action in the latter clause. Feels like “once it comes to it, we must…”.
Scope of use: both hypothetical situations (もし～) and those nearly certain/already decided (いざ/いよいよ～). The latter clause is often a plan, preparation, or a predicted consequence.
Compared with ～となると/～となれば: となったら is more casual, pragmatic, and often used with personal preparations; となると tends toward “if/when it comes to that…” with evaluation/difficulties; となれば leans toward broad hypotheticals, more written-style.

留学するとなったら、早めに準備を始めるべきだ。
If you’ve decided to study abroad, you should start preparing early.
いざ出発となったら、急に不安になってきた。
When it finally came time to depart, I suddenly got anxious.
引っ越しとなったら、粗大ごみの処分が大変だ。
If it comes to moving, disposing of bulky garbage is a hassle.
もし大会が中止となったら、参加費は返金されます。
If the tournament is canceled, the entry fee will be refunded.
来週となったら、もう間に合わないかもしれない。
Once it gets to next week, it might be too late.

Use when: referring to the point when a decision/event starts (本番、締め切り、出発、中止...), or when an action has been set in motion (V-辞書形).
The latter clause often goes with: 準備する、手配する、覚悟する、～べきだ、～つもりだ、～ないと（いけない）.
Has a stronger decisive/personal tone than となると; more colloquial and familiar than となれば.
Sounds unnatural if the latter clause is a direct command to the listener in formal contexts; prefer a suggestion/milder volition.

Pattern	Meaning	Main difference	Short example
～となったら	If/when it becomes A...	Leans toward personal intent/preparation; casual	引っ越すとなったら、段取りを決めよう。
～となると	If/when it comes to A...	Raises objective issues/consequences; shifts the tone of the discussion	雨となると、計画は見直しだ。
～となれば	If/supposing it comes to A...	Broad hypothetical, written style, strong sense of inevitability	中止となれば、損失は大きい。
～としたら/～とすれば	If we suppose/assume A...	Purely hypothetical condition, neutral	行くとしたら、いつにする？

“いざ～となったら/となると” is a very natural set phrase to stress “when it actually comes to it.”
With time markers: 今日・明日・来週・年度末 + となったら expresses “once it gets to…”.
Intonation: the となった「ら」 softens the sentence compared to となると; gives a post-decision feel.
When A is an official announcement (会社の方針、中止等), となったら shows how the speaker will “respond/act.”

Mixing it up with となると: When the sentence highlights objective issues, となると is more natural (e.g., お金がかかる “となると” sounds natural).
Using with adjectives: Aい/na + となったら is less natural; prefer event-like N/V.
Direct commands: 避ける with となったら in formal contexts; replace with ～べきだ／～ほうがいい.
JLPT: Distinguish personal tone (となったら) vs. objective (となると) vs. broad hypothetical (となれば).