1·He got a large piece of cardboard, painted a sign begging for the bicycle to be returned, and tied the sign to the letterbox.
他拿了一大块硬纸板，画了一个乞求归还自行车的牌子，然后把牌子系在了信箱上。
2·In the current housing slump many American homeowners have resorted to "jingle mail", dropping their keys through the lender's letterbox and walking away from their property.
在如今美国房产市场衰退的情况之下，许多房主采用一种非常简单的方式：将房屋的钥匙投入出借人的信箱之中，然后离开他们的房产。
3·Peep through the letterbox and see if anyone is at home.
透过信箱看家中是否有人。
4·I think applying a Letterbox resizing is using matrix transformation, so even if your image appears larger, its position and size are still the same.
我想申请一个信箱大小是使用矩阵变换，所以即使你的图像显得更大，其位置和大小都一样。