Mr. Yu( )us maths since 1982.请解释一下为什么填has taught, 谢谢！

has taught 是现在完成时，表示已经完成的动作.

这句话的意思是：于先生从1982起一直教我们数学。

has been doing 是现在完成进行时

has done 是现在完成时

现在完成进行时的谓语动词构成

I/ we/ you/ they have been doing sth. he/ she/ it has been doing sth.

（一）表示从过去某时开始一直持续到现在的动作，并且还将持续下去。

The Chinese have been making paper for two thousand years. 中国有2000年的造纸历史。（动作还将继续下去）

I have been learning English since three years ago. 自从三年前以来我一直在学英语。（动作还将继续下去）

（二）表示在说话时刻之前到现在正在进行的动作。

We have been waiting for you for half an hour. 我们已经等你半个钟头了(人还没到，如同在电话里说的，还会继续等)

（二）有些现在完成进行时的句子等同于现在完成时的句子。

They have been living in this city for ten years. They have lived in this city for ten years. 他们在这个城市已经住了10年了。

I have been working here for five years. I have worked here for five years. 我在这里已经工作五年了。

（四）大多数现在完成进行时的句子不等同于现在完成时的句子。

I have been writing a book.（动作还将继续下去）我一直在写一本书。

I have written a book.（动作已经完成）我已经写了一本书。

They have been building a bridge. 他们一直在造一座桥。

They have built a bridge. 他们造了一座桥。

（五）表示状态的动词不能用于现在完成进行时。

I have known him for years. 我认识他已经好几年了。

* I have been knowing... 这类不能用于现在完成进行时的动词还有：love爱，like喜欢， hate讨厌，等。