COULD / WAS -WERE ABLE TO 

 we use then to talk about "PAST ABILITIES"

PRESENT                                   PAST        

                                                  can                                         could
                                                  am                                         was
                                             is able to                                     were    able to
                                                 are

EXAMPLE:

they were able to reach a rock.
the children were able to climb the mountain.
he was able to rescue the children from the water.
i was able to swim very fast.
you could see the top of the mountain.

STRUCTURE:

subject   +   coul/was-were able to   +   verb   complement