"Mail a postcard" 是邮寄一张明信片用英语的表达方式。
来源： mail 是英语中邮寄的意思， postcard 是明信片的意思。
例句：
"I'm going to mail a postcard to my friend in France." (我要给我在法国的朋友邮寄一张明信片。)
"Can you please mail this postcard for me?" (请你帮我邮寄这张明信片好吗?)
"I bought a postcard at the souvenir shop and I'm going to mail it to my family." (我在纪念品店买了一张明信片，我要邮寄给我的家人。)