Auxiliar  modal could (podía, podrìa)en forma afirmativa,negativa e interrogativa 

 Afirmativa

Sujeto+could+verbo+complemento

                  My grandmother could speak english

Negativa

Sujeto+could not/(couldn't)+verbo+complemento

          My grandmother couldn't   speak english

Interrogativa

Could+sujeto+verbo+complemento+?

               Could my grandmother speak english?

Using the auxiliary could: a possibility in the past or a skill we use the auxiliary Could you talk about that action was possible in the past. In addition we were able to do or perform freely. Could help us (positive) and Could not (negative). For overall skill happened in past