one / ones 代替前面提过的不定名词以避免重复(某一个)
"I bought a digital camera yesterday."「我昨天买了一个数字相机。」
"Can I have a look at it? I want to buy one, too."
"Do you have a knife? I need one to sharpen my pencil."
"Yes, I have one. It is in my pencil case."
"Don't forget your lunchbox!" "I've got it."
"These apples are too small. Do you have bigger ones?"
"The bigger ones are all sold out. We have only small ones left now." 「大的都卖完了，现在只剩下小的。」内容来自 听力课堂网：http://www.tingclass.net/show-5855-219689-1.html