When did you receive that letter?为什么句子中有一个that

问题描述:

When did you receive that letter?为什么句子中有一个that
教教我

那封信.应该之前有提到过那封信.什么意思,不明白。that 是那个的意思。
是一种特指。
所谓特指,就是前文提到了,这里特指前文提到的letter.