One day, the father lets eight year-old son send a letter, 
有一天,父亲让八岁的儿子去寄一封信,

the son took the letter ,
儿子已经拿着信跑了

the father then remembered didn't write the address and addressee's name on the envelope.
父亲才想起信封上没写地址和收信人的名字。

After the son comes back, the father asks him: "You have thrown the letter in the mail box?"
儿子回来后,父亲问他:“你把信丢进邮筒了吗?”

"Certainly"“当然”

"You have not seen on the envelope not to write the address and the addressee name?"
“你没看见信封上没有写地址和收信人名字吗?”

"I certainly saw nothing written on the envelope."
“我当然看见信封上什么也没写”

"Then why you didn't take it back?"
“那你为什么不拿回来呢?”

"I also thought that you do not write the address and the addressee,“
我还以为你不写地址和收信人,

is for does not want to let me know that you do send the letter to who!"
是为了不想让我知道你把信寄给谁呢!”