I had a letter ---(send)in the post office,I was too busy.

问题描述:

I had a letter ---(send)in the post office,I was too busy.

to send

sent

sent
have sb done sth 使某人做某事
have sth done 使某事被做
要用 过去分词

sent
had a letter sent 叫别人帮我寄信
have sth done句型,叫某人做某事