1） There are three reasons for this.
2） The reasons for this are as follows.
3） The reason for this is obvious.
4） The reason for this is not far to seek.
5） The reason for this is that...
例如：There are three reasons for the changes that have taken place in our life. Firstly， peoples living standard has been greatly improved. Secondly， most people are well paid， and they can afford what they need or like. Last but not least， more and more people prefer to enjoy modern life.
1） It has the following advantages.
2） It does us a lot of good.
3） It benefits us quite a lot.
4） It is beneficial to us.
5） It is of great benefit to us.
例如：Books are like friends. They can help us know the world better， and they can open our minds and widen our horizons. Therefore， reading extensively is of great benefit to us.
1） It has more disadvantages than advantages.
2） It does us much harm.
3） It is harmfulto us.
例如：However， everything dividesinto two. Television can also be harmful to us. It can do harm to our health and make us lazy if we spend too much time watching television.
1） It is important （necessary， difficult， convenient， possible） for sb. to do sth.
2） We think it necessary to do sth.
3） It plays an important role in our life.
例如：Computers are now being used everywhere， whether in the government， in schools or in business. Soon， computers will be found in every home， too. We have good reason to say that computers are playing an increasingly important role in our life and we have stepped into the Computer Age.
1） We should take some effective measures.
2） We should try our best to overcome（conquer）the difficulties.
3） We should do our utmost in doing sth.
4） We should solve the problems that we are confronted（faced）with.
例如：The housing problem that we are confronted with is becoming more and more serious. Therefore， we must take some effective measures to solve it.