able to點用?

be able to=can=have the ability to do something=有能力去做一些事情
have been able to 和 will be able to 只不過是不同的tense
have been able to = 現在完成式的"be able to"
will been able to=錯的，應該是will be able to ，是future tense
unable to = not able to= 不能做到。

here are some examples:

1)Don't you know that I have been able to speak Spanish for ages? My boyfriend is Spanish!

2)If I can get a job and earn some money, then I will be able to go to Switerland this Easter.

3)You are still young, but I'm sure one day you will be able to understand what I have just said.

4)I am unable to take care of myself because I just lost my arms in a car accident.