Cuando se trata del pasado de can, la opción automática es usar could. Sin embargo, hay ocasiones cuando could no vale y hay que usar was able to / were able to.
En la mayoría de casos se pueden utilizar tanto could como was able to / were able to.
They could help us (when we were young). / They were able to help us (when we were young). > Podían ayudarnos (cuando eramos jovenes).
En el pasado, could (could do, could go, could see, etc.) en positivo representa posibilidad o capacidad, pero no el éxito de un evento puntual: was able to o were able to representan este concepto.
They were able to help us (yesterday). > Pudieron ayudarnos (ayer).
No hay diferencia entre could y was able to / were able to en forma negativa.
They couldn’t help us. / They weren’t able to help us. > No podían ayudarnos. / No pudieron ayudarnos.
