At a glance
～とすれば means "if we assume that...", "supposing that..." or "if it is true that...". You set up a supposition, then state the conclusion that follows from it.
[Verb / い-adj plain form] とすれば、[conclusion]
[Noun / な-adj] だ とすれば、[conclusion]
その話が本当だとすれば、大変なことだ。 If that story is true, it's a serious matter.
How to form it
とすれば attaches to the plain form. Past and negative plain forms also work.
| Attaches to | Present | Negative | Past |
|---|---|---|---|
| Verb | 行くとすれば | 行かないとすれば | 行ったとすれば |
| い-adjective | 高いとすれば | 高くないとすれば | 高かったとすれば |
| な-adjective | 便利だとすれば | 便利じゃないとすれば | 便利だったとすれば |
| Noun | 学生だとすれば | 学生じゃないとすれば | 学生だったとすれば |
| Start of sentence | だとすれば / とすれば | "If that's the case" |
Register: logical and a little formal. It is common in writing, discussions and careful speech.