This is the factory where we visited last week.

把where改成that 或者改成which
“这就是我们上周参观过的那个工厂.”
关联词后面的部分是定语从句,关联词代替的是“the factory”
where后面引导地点状语从句.
例如：this is the factory where they worked.
“这就是他们曾经工作过的那个工厂.”
where代替的是“in the factory”