Definition: a flat paper container for a letter Level: A2
他正在递出一个信封。
He is handing over an ___envelope.
请把信放进信封里再寄出去。
Please put the letter in the ___envelope before sending it.
我在抽屉里发现了一个信封，上面写着一个地址。
I found an ___envelope with an address written on it in the drawer.
