The fire spread through the hotel very quickly but everyone was able to get out.
本句不可以用 could 替换 was able to，因为 but 分句暗示：酒店里面的所有人都已成功逃脱。
could do sth. 表示 “(过去) 有能力做某事”，was able to do sth. 除了这层意思外，还有一个重要的意思，就是：“设法做了某事(managed to do sth.)”，“成功地做了某事(succeeded in doing sth.)”。如果只表达“有能力做某事”这层意思，那么这两个可以互换，而意思保持不变，例如：
He was very strong and he could (=was able to) lift that big stone. 他很强壮，他能够举起那块大石头(意思是他有这个能力)。
但是，如果是第二层意思，那么，只能用 was able to，不能用could，例如：
At last he was able to solve the problem.（正）
At last he could solve the problem.（误）
Only when the war was over was he able to get happily back to work.（正）
Only when the war was over could he get happily back to work.（误）
但是，当句子是否定意思时，could 和 was able to 没有差别：
I couldn’t persuade him to sign the agreement. = I wasn’t able to persuade him to sign the agreement.
