你就说：
when they found the girl, she was buried under the ruins of a church.
或者 when she was found, the girl wasburied under the ruins of a church.

语法分析：

the poor girl （主语）was found （谓语）buried （主语补足语）under the ruins （地点状语）
-------这是“被动语态”

--------（主动语态）：they (主语)found（谓语） the girl （宾语）buried （宾语补足语） -- --


祝你你教学顺利!