如何表达和计算英文的discount例如打7折,如何用英语表达；再例如货品的总值1000,现给予25%的discount ,又如何计算.我虽要对折扣详尽的解释

Discount 在中英文中的计算不一样。
中文表达：例如打7折，原价为1000RMB, 则 折扣后的价格为： 1000*0.7=700RMB，实际便宜300人民币;
英文中:e.g. 30% down（打七折）, origianl sales price is 1000RMB, then the price after discout is : 1000*0.3=300RMB, you save 700RMB actually.
一句话，外国人的70% down， 意味70%的原价折掉，你只需付原价*(1-0.7)的价格即可。

30% off or 30% discount
Calcoulation is 1000*(1-25%)=750
I will give you 25% discount,that will be 750.
Please notice,discount has another meaning in Accounting--贴现,折现
This is more professional and will help you earilier to understand the meaning of "discount".
According to your question,I will give you 25% DISCOUNT to make you buy my goods.So the cost of mine is 25% discount(250)
Such example please refer following:
We give 10 percent discount for cash.
现金付款,我们九折优惠.