If that was all there were to the lesson, it would be a hands-on class in art history. 

【翻译】如果说这就是该课的全部内容，那将是艺术史上的一堂实践课。

there were 是定语从句，修饰all，省略了关系代词 that。我们知道，there be 是存在句，表示“所存在的”的意思，当它作定语修饰all的时候，说明“存在的、所有的一切”。

例如：

1. She's going through a difficult time. That's all there is to it.  

她正经历一个困难时期。就这么简单。

来自柯林斯例句

2. I have said all there is to be said about it.  

关于这件事,该说的我都说了。

来自《现代汉英综合大词典》

3. That's all there is to it!  

如此而已,岂有他哉!

来自《现代汉英综合大词典》

4. One can never learn all there is to know about cooking, and that is one of the joys of being a chef.  

钻研烹饪之道是永无止境的，这也是当厨师的乐趣之一。

来自柯林斯例句

5. That's about all there is. Unless we change our minds and go downtown.  

差不多就这么多了. 除非我们改变主意去市区。

来自休闲英语会话

6. Love is all there is; it makes the world go round.  

爱情就是一切, 它令地球旋转。

7. To live is to function. That is all there is in living.  

活着就要发挥作用, 这就是生活的全部内容。