Complete the detective's deduction about a notoriously clumsy cat burglar. Select the correct phrase to fill in the blanks.

If the thief ________ so clumsy by nature, he ________ over the alarm wire and triggered the siren last night.

The correct answer is weren't / wouldn't have tripped.

This mixed conditional combines a present/ongoing unreal condition with a past unreal result. Because the thief is clumsy "by nature" (an ongoing trait), we use the past subjunctive "weren't". Because the tripping happened "last night" (a past event), we use "wouldn't have" + past participle.