例句：

　　So energy can go back and forth.

　　所以能量可以來(lái)來(lái)往往。

　　Back and forth I went with the slides.

　　于是，我就來(lái)來(lái)回回地放著幻燈片。

　　The buoy floated back and forth in the water.

　　浮標(biāo)在水里漂來(lái)漂去。

　　Dogs always bark at strangers.

　　狗總是對(duì)著陌生人吠。

　　My neighbor's dog bark at me.

　　鄰居的狗對(duì)我吠叫。

　　The pet doesn't bark at night.

　　那寵物在夜里不叫。

　　I'm afraid I won't be able to do that.

　　恐怕我做不了這件事。

　　I need time to be able to do my homework.

　　我需要時(shí)間來(lái)做作業(yè)。

　　What would you like to be able to do next?

　　你下一步想做什么?