Linda is right. But you can use "able" or "can't" for both of the expressions. It's not like you can only use 'able' with certain words or anything. So all of the following is fine: - I can't ride a bike. - I'm not able (unable) to ride a bike. - I can't speak English. - I'm not able (unable) to speak English. "Can't" is used most of the time though as it's less formal than "to be able". So I think you might want to stick to using "can't" if you want to express that you can't do something.

I cannot (can't) ride a bike. or I'm not able to speak English.