COULDN´T / WASN´T-WEREN´T ABLE TO 

they are negative and are use to talk about past abilities.

EXAMPLE:

• I  couldn´t dance salsa at five years old.
• they couldn´t survive.
• we weren´t able to find water.
• the children weren´t able to turn in the right way home.

QUESTIONS:

could                                         were 
was        +       subject      +      able to      +   comp.?
were

EXAMPLE:

could you swim very fast?
yes, i could.
no , i couldn´t

were i could able to run fast?
no , they weren´t.