We use “able” especially in the structure be able +infinitive. This often has the same meaning as can. The negative form is unable.
- Some people are able/can walk on their hands
- I am unable/can’t understand what she wants
Can is preferred in the sense of “know how to” and in expressions, like can see, can hear..etc
- Can you knit? / I can see a ship
Be able is used in cases where can is not grammatically possible (future/present perfect, conditional…)
One day scientists will be able to find a cure for cancer (NOT… will can find)