Were it not for ~ ⇔ If it were not for ~ ⇔ But for ~ ⇔ Without ~

現在の仮定：If it were not for ⇔ Were it not for ⇔ But for ⇔ Without。過去の仮定：If it had not been for ⇔ Had it not been for ⇔ But for ⇔ Without。if を省略すると were / had が文頭に出る。

• Were it not for water, no living things could survive.
  水がなければ、どんな生物も生きられないだろう。
• If it were not for water, no living things could survive.
  もし水がなかったら、生き物は生存できないだろう。
• Without water, no living things could survive.
  水なしでは、いかなる生物も生き延びられないだろう。
• Had it not been for your help, I would have failed.
  あなたの助けがなかったら、私は失敗していただろう。

直説法に戻すと Because there is water, living things can survive. ／ Because you helped me, I succeeded. となり、実は「水・あなたのおかげ」という感謝や重要性を強調する構文。選択肢では thanks to ~ に言い換えられる。