过去式是backed，第三人称单数是backs，现在分词是backing，例如：

1、I'm having treatment for my back problem.我正接受背部疾患的治疗。

2、New skin had to be grafted on from his back.需从他的背部移植新皮肤。

3、There's room for three people in the back.后排有可容三人的空位。