开头：

It is often said that ... , 据说……/人们常说……

As the proverb says ... , 古人云……

It goes without saying that ... ,

It is clear/obvious that ... ,

Many people often ask ...

扩展：

It is true that ... ,

Everybody knows that ... ,

It can be easily proved that ... ,

No one can deny that ... 不可否认……

There is no denying that ... 不可否认……

The reason why ... is that ... ,

There is no doubt that ... ,

To take ... for an example (instance) ... ,

We know that ... ,

What is more serious is that ...

I do not believe that ... ,

Perhaps you'll ask why ...

That's why I feel that ...

结论：

in a word, in general, in short, above all, after all, generally speaking, finally, in conclusion, at last, in summary, as a result, above all, thus, eventually, on the whole（就整体而言）,

From this point of view ...

On account of this we can find that ...