1. 正在打折的英语表达是 "On sale" 或 "Now on sale"。

2. "正在打折的商品" 可以用 "Items currently on sale" 来表达。

3. 当我们想要说某个商店正在打折促销时，可以说 "The shop in front of us is having a discount promotion."

4. 关于星星的数量，古希腊人的比喻是说星星和尼罗河的沙子一样多，这个表达可以译为 "An ancient Greek once said that the stars are as numerous as the sand in the Nile River."

5. 如果要表达“商品都正在打折”，可以说 "All goods are on sale."

6. 英语中表达“正在销售”可以说 "In stock" 或 "For sale"，而“打折”则是 "On sale" 或 "Discounted."

7. 打折的英语表达可以是 "Discount" 或 "Sale," 具体如 "15% off" 表示的是折扣。

8. 如果想要表达“今天正好打折”，可以说 "It just so happens that there is a sale today."