Podia vs pude

Podia vs pude
 

could they buy the tickets yesterday? yes they were able to buy them. I have a hard time understanding when to use could in preterite and imperfect. Can someone give me me an example? por favor empleando poder. podia vs. pude etc.

¿Pudieron comprar las entradas ayer? Sí, las pudieron comprar.

@tango: You may want to take a look here, here, here or here, for examples on using imperfect and past. :)