The professor suggested the problem referred to be solved at once.

题目

The professor suggested the problem referred to be solved at once.
solved为什么不用ing形式呢?

答案

The professor （主语）suggested （谓语）.
宾语从句
the problem（主语） referred to（=which had been referred to是定语）（should 省略了） be solved at once
去掉修饰语
.（suggested) the problem (should) be solved(被动语态）
suugest "建议“ 接句子时用虚拟语气（should)+动词原形
全句意为 “教授建议所提到的那个问题应该马上予以解决”
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