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当从句中有were/had/should时，可省略if，而把他们放在句首
步骤：（1）去掉If（2）三词提前（3）主句不变
E.g.：1. If I were a billionaire, I would eat crabs everyday→ Were I a billionaire, I would eat crabs everyday .
2. If I had done well in the English exam, I would have entered Beijing University in 1990.→ Had I done well in the English exam, I would have entered Beijing University in 1990.
3. If I were to meet Lancer tomorrow, I would give him some advice.→ Were I to meet Lancer tomorrow, I would give him some advice.