"Buy one, get one free" is a common way to say "买一送一" in English. This phrase originated as a promotional tactic used by retailers to encourage customers to make a purchase. For example, a store might offer a deal where if a customer buys one item, they will receive a second item for free. This can be a great way for customers to save money, and for retailers to increase sales.

Here are some examples of how "buy one, get one free" might be used in sentences:

"If you buy one pair of shoes, you can get a second pair for free." (买一双鞋子，可以免费获得第二双。)

"This week, we are offering a buy one, get one free promotion on all of our books." (本周，我们所有的书都提供买一送一的促销。)

"Don't miss out on our buy one, get one free deal on pizza - order now and get twice the pizza for the same price!" (不要错过我们的买一送一的披萨促销 - 现在下单，同样的价格可以得到两倍的披萨！)