In English, "could" is both past tense and conditional tense of "can". If you look at the English sentence, you can figure out if it refers to the past or not. If it does, you should use "pude/podía", if it doesn't, you want to use "podría" (in some cases "pudiera/pudiese").
For example:
Ten years ago I could run for almost 2 hours, but now I can't. (It's a clear past tense). --> Hace diez años PODÍA correr durante dos horas, pero ahora ya no puedo.
Could you please pass me the salt? (It doesn't refer to the past) --> ¿PODRÍAS pasarme la sal, por favor?
In most cases, this tip should help you:
--> If you can substitute "could" by "was/were able to", use the past preterit/imperfect.
--> If you can substitute "could" by "would be able to", use the conditional.
If you have any questions, don't hesitate to ask!!
Good luck :)
Fer