mail a letter：发一封邮件
e.g. How much does it cost to mail a registered letter to China?
寄往中国的挂号信要多少钱？
e.g. They asked her to mail him a letter.
他们要她给他寄封信。
e.g. He went to the post office to mail a letter.
他到邮局寄信去了
