Having received a latter, I knew everything is all right.为什么having用现在进行时?

英语中一个句子是不能有两个谓语动词的，这样其中一个要用分词形式或从句形式。

having received a letter就是I had received a letter的分词形式，是完成时，整个句子相当于：
After I had received a letter, I knew everything was all right.
因为receive a letter这个动作发生在过去的过去,用过去完成时，转变成分词形式是：having received a letter.

动名次做主语啦，祈使句