The school where he once studied is very famous.

这句话需要倒过来看,我们英语老师教我们这样看的
例：the school _____ he once studied.
the school _______ he once visited.
看第一句：正过来说,he once studied in the school,有 in ,所以用where
看第二句：正过来说,he once visited the school,没有 in,所以用that/which
当然,如果句中已经有了in,那就用that/which
这是绝对准确的,谢谢支持!
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