题目内容：

the student saw something strange ______when they were walking near the river
a happen b happens c happened d to happen

优质解答

A
see sb do sth
happen 是不带to 的不定式做宾补

追问：

那可不可以用happened？这样就是宾语从句啊？

追答：

如果用从句， 由于此动作先于主句中的谓语动词saw 发生要用过去完成时 had happened