but和however表示对比，常常用来提示重要信息，例如：The rising birth rate is not due to increased fertility, but to a sharp decline in the death rate.
在文中提问可以突出问题后的答案，提示考生答案中有重要信息，例如：Why is a piped water supply so important? Disease due to contaminated water is a common cause of death in childhood.
有时，为了保证读者完全理解自己的观点，作者会在文中反复提出自己的观点，例如：Death control can be achieved autonomously. In other words, the death rate can be cut without anything else changing.
TIPS：让写作增色的好句子
Time flies.时光易逝。
One today is worth two tomorrows.一个今天胜似两个明天。
When an opportunity is neglected, it never comes back to you.机不可失，时不再来。
Strike while the iron is hot.趁热打铁。
Wisdom is more to be envied than riches.知识可羡，胜于财富。
It is good to learn at another man's cost.前车可鉴。
Seeing is believing.百闻不如一见。
