Modal Verbs: Can - Could - Was able to

Can
The modal verb can is used to express:

• the ability to do something: I can speak English

• the possibility to do something: This evening i can go out, because tomorrow is my day off

• to say that something is probable: If you don't pay attention on the stairs, you can hurt yourself 

• expresses the permission to do something: You can take my car this weekend if you need it

Could 
The modal verb could is used to:

• express the conditional in sentences as the conditional form of can: If I had more time, I could travel around the world

• express possibility: You could to Rome by train, rather than the car.

• express past ability as well: I could run ten kilometers in one hour, when I was younger.

• make suggestions: You could spend your holidays in my house in Toscany, if you like.

• make polite requests: Good evening, Could I have something to drink? 

As we have specified above could is used to for general ability, but if you want to say that somebody did something in a specific situation, is good to use the form:
was/were able to or managed to

examples:
- You will be able to speak perfect English very soon. 
- I would like to be able to speak 5 languages.
- I have been able to ride the bicycle since I was 6.