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...
6)We have good reason to believe that...
There are three reasons for the changes that have taken place in our life.Firstly，people’s 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.
注：如果写第一个句子没有把握，可将其改写成两个句子。如：Great changes have taken place in our life. There are three reasons for this.这样写可以避免套用中的表达失误。
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.
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 harmful to us.
However，everything divides into 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.