例句：
So energy can go back and forth.
所以能量可以来来往往。
Back and forth I went with the slides.
于是，我就来来回回地放着幻灯片。
The buoy floated back and forth in the water.
浮标在水里漂来漂去。
Dogs always bark at strangers.
狗总是对着陌生人吠。
My neighbor's dog bark at me.
邻居的狗对我吠叫。
The pet doesn't bark at night.
那宠物在夜里不叫。
I'm afraid I won't be able to do that.
恐怕我做不了这件事。
I need time to be able to do my homework.
我需要时间来做作业。
What would you like to be able to do next?
你下一步想做什么?