the word "can" is best used in place of the verb "to be able" I am able to run a mile I can run a mile She is able to ride a motorcycle She can ride a motorcycle It is the same for all subjects (I, you , he/she, we, they) They can... We can... It can... You would only use the verb "to be able" pretty much in the future tense I will be able to fly an airplane She will be able to speak English very soon We will be able to go to Mars in the future

"When do I have to use the expression 'be able to'?" Sometimes, you can use "can" or "could" instead of "be able to",but remember that the use fo "can/could" is limited because they are modals. For example: "I will can finish the work" - IMPOSSIBLE. You can't make two modals modify one verb (finish). "I will be able to finish the work." - perfectly fine. "I have can finished the work" - again, IMPOSSIBLE. Modals work as helping verbs, and in this example "have" is also a helping verb (present perfect). You cannot have a modal follow a helping verb. "I have been able to finish the work" - perfectly fine. So, as you see "be able to" is far more flexible than "can/could" because you can conjugate "be" easily. Still, you need to know how to use both.

You can also use BE ABLE TO to describe an ability in the present or future but can is use more frequently for present tense. Ex: I will be able to speak English in couple of months. Hopefully you understand...:)

I think you mean instead of "can" or "could" or - right? In that case, we use "be able to" when we talk about past opportunities. I was able to visit Malaysia last month. (Not, "I could visit Malaysia last month.") We also use it to talk about current and future skills/abilities and possibilities. I am able to type at 80 words per minute. (more formal than "I can type at 80 words per minute.") Would you be able to work next Saturday? (more formal than "Could you work next Saturday?")