对过去的虚拟结构中, could have 有时被用来指能力:

If I could have gone with you, I would have.

 ( = If I had been able to go with you, I would have gone.)

If she could have done the work, she wouldn't have needed help. 

( = If she had been able to do the work, she wouldn't have needed help.)

If I could have left work early, I would've been very grateful. 

( = If I had been able to leave work early, I would've been very grateful.

## could have既然已经是表示能力，再用“ if sb could have been able to" 理论上应该是重复的。然后是结构太复杂了，估计没几个人这样用。