Reasoning From an Assumption
とすれば takes something as given and asks what follows from it.
- 彼が来ないとすれば、誰が司会をするのか。(Supposing he doesn't come, who runs the meeting?)
This is a thinking pattern, not an event pattern. The second half is a deduction, a question or a judgement — not something that simply happens.
The Three Variants
| Form | Lean |
|---|---|
| とすれば | logical deduction |
| としたら | a hypothetical situation, slightly more spoken |
| とすると | a natural inference from what was just said |
The differences are slight and most sentences accept all three.
Against たら and なら
| Pattern | Sense |
|---|---|
| たら | when/if it happens — an event |
| なら | if that's the case — picking up the topic |
| とすれば | supposing it's true — reasoning it through |
雨が降ったら中止です is a plan. 雨が降るとすれば準備が要る is working out consequences.
もし and 仮に
Both often open the sentence to flag the hypothesis: 仮に失敗したとしても… (even supposing it failed…).