第一类：表示原因

　　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...

　　第二类：表示益处/害处

　　1) It has the following advantages.

　　2) It does us a lot of good to ...

　　3) It benefits us quite a lot.

　　4) It is beneficial to us.

　　5) It is of great benefit to us.

　　6) We have good reasons to believe that...

　　7) It has more disadvantages than advantages.

　　8) It does us much harm.= it does much harm to us.

　　第三类：表示变化

　　1) Some changes have taken place in the past five years.

　　2) A great change will certainly be produced in the world's communications.

　　3) The computer has brought about many changes in education.

　　第四类：表示重要性

　　1) Doing sth. is of utmost importance.

　　2) It plays an important role in our life.

　　3) It is vital that .../ to do sth

　　4) Sth plays an essential role/part (in sth).

　　5) Sth is an essential part of sth.