The teacher said to his students,''Tell briefly in your own words how penicillin was discovered.为什么The teacher这句改为直接引语后was会变成had been变成had

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The teacher said to his students,''Tell briefly in your own words how penicillin was discovered.
为什么The teacher这句改为直接引语后was会变成had been
变成had

因为主句是一般过去时( SAID) ,从句要用相应的过去时( 原来是WAS DISCOVERED 一般过去时,所以要改为相应的过去时,即过去完成时 hade been discovered )
The teacher told his students to tell briefly in their own words how penicillin had been discovered.

直接引语后was会变成had been,因为这是直接用语变成间接用语固定语言规则