Question

The other day, my brother drovehis car down the st

The other day, my brother drovehis car down the street at ____ I thought was a dangerous speed. 为什么不能用which而用what？

Answer 1

你好！

首先这里应该填what
假设填的是which,那么at which引导的是定语从句，，先行词是street，那从句就成了“我觉得这条街是个很危险的速度。” 句子意思不同，故不能用which

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Answer 2

what 引导从句时，相当于 the thing that，包括先行词+连词

如果用which，那么就没有先行词。
如果用which，应该改成：
The other day, my brother drovehis car down the street at a speed which I thought was a dangerous speed.

Answer 3

what 引领的是主语从句，而which 不能。