Although the soldier was badly wounded, he was able to tell what had happened. She ran fast but she couldn’t /wasn’t able to catch the bus.

3.表示特定的某一过去能力或表示成功地做了某事时，只能用was/were able to， 不能用could。

He was able to flee Europe before the war broke out.    He was able to swim halfway before he got tired. 　　

表示过去的能力