They love it when the post arrives.

love为及物动词，必须有宾语，但love又不能接宾语从句。如果想要表示他们喜欢邮件到达的时候这个意义时，不能用when从句作love的宾语。于是用虚指it占据宾语的位置，再用when从句作时间状语从句。动词like, hate, take, appreciate都有类似用法。例如：

I’ll appreciate it if you can do me a favor.

我喜欢这里，英文是I like it here. 而不是I like here. 副词here不能做like的宾语。

He wants it to be his turn. 这是复合宾语结构, 不定式为宾补。

It is not his turn, but he wants it to be his turn.