be able to 能够
a.位于助动词后.
b.情态动词后.
c.表示过去某时刻动作时.
d.用于句首表示条件.
e.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 managed to flee Europe before the war broke out.