Can, could and be able to

"Can", "Could" and "be able to" are used to express ability 
"Can't", "Couldn't" and "not be able to" are used to express inability.

Use of can/can't: FOR THE PRESENT.

I can swim, play the violin and ride a bike.
I can't speak Catalan. I don't know a word in Catalan.

Use of could/couldn't: FOR THE PAST

I could sing very well when I was a child.
I couldn't understand Catalan before I took a course.

Use of BE ABLE TO: FOR ALL VERB TENSES.

"Be able to" in the present or past is more FORMAL than "can" or "could"
Just use the correct form for each verb tense:

I'm able to climb that mountain.
He wasn't able to ride a bike when he was a child.
Will he be able to go to Catalonia next year?
I like being able to study what I like.

EXERCISES

 Can or could: grammar + exercise

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