1. Buy one, get one free.
2. Buy one, and get one free.
3. If you buy one, you will get one free.
4. If you buy one, you get one free.
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What does #1 mean?
If a store is having a sale on shoes, #1 (and all the others) mean that if you want to buy two pairs, you'll pay only for one pair.
... or bottles of shampoo or shirts or whatever the store is selling (get two but pay for only one).