It has been estimated that if the bridge were packed with cars, it would still only be carrying a third of its total capacity.

However, size and strength are not the only important things about this bridge.

1. 上面第一句不是描述的是事实吗？不应该用现在时吗？

2. 第二句中的 about 能不能换成 of？

解答如下：

同意曹老师观点，以上描述属于一种无法完成/实现假设，必须使用虚拟语气，关于介词，应该使用about 不适合用介词of替换。介词of是直接针对后面事物，介词about表示间接针对与，关于.

(1)of prep.  关于; 属于…的; 由…制成

This is a photograph of my dog.
这是一张我的狗的照片。

He is a friend of my father.
他是我父亲的朋友。

She did it of her own will.
她是自愿这么做的。

He felt quite certain of success.
他对成功很有把握。

Tell me all about it.
告诉我关于这件事的一切。

(2)about 关于;对于;涉及;在…方面

All about us in the store were piles of goods.
在这家店里,我们周围到处都是成堆的商品

She came in for a coffee, and told me about her friend Shona.

她进来喝了杯咖啡，并向我谈起了她的朋友肖纳。

She knew a lot about food.她对食物很在行。

He never complains about his wife. 他从不抱怨妻子。

Leadership is about the ability to implement change.  

领导才能就是要有实施变革的能力。