Be able to 的否定句

I'm able to attend the meeting. 我能参加会议。—> 
I am not able to attend the meeting.
我不能参加会议。

You're able to leave the country without a passport. 你可以在没有护照的情况下离开这个国家。—>
You aren't able to leave the country without a passport. 
没有护照你不能离开这个国家。

He's able to drive a car. 他会开车。—>
He isn't able to drive a car.
他不会开车。

She's able to read without glasses. 她不戴眼镜就能看书。—>
She isn't able to read without glasses.
她不戴眼镜就不能看书。

We're able to buy a new house. 我们能买个新房子。—>
We aren't able to buy a new house.
我们买不起新房子。

They're able to walk very far. 他们能走很远。—>
They aren't able to walk very far.
他们不能走很远。

Be able to 的过去时

把 be 变成 was/were —> was/were able to
I was able to come to your party.
[过去某时]我能来参加你的聚会。
I wasn't able to come to your party.
我没能来参加你的聚会。

We were able to leave on time.
[过去某时]我们能够准时离开。
on time 准时
We weren't able to leave on time.
我们没能准时离开。