Cuando quieres indicar que no tienes conocimiento de algo y que nunca lo has tenido se utilizaría no sabía. Ejemplo Viaje en el tren con 3 alemanes y como no sabía hablar aleman estuve callado durante todo el viaje. Mientras que se utiliza "no supe" relacionado con no saber algo en un momento determinado. Ejemplo Vi a María y no supe que decirle. Quiere decir que en ese momento no fuiste capaz de hablar con ella. Haciendo hincapie (en ese momento) que en cualquier otro momento si que hubieras sabido que decirle pero en ese momento no.

Sean, Spanish has two forms for past. These are known as "perfective" and "imperfective". The perfective form means that the action has a limit (most grammarians say that perfective means that the actions has been finished but many linguist, including me, do not agree with that, it is more accurate to say that the action has limitations or boundaries). Imperfective means that the action is extended in the time or that the same action was repeated over and over again. For example: Te llamé pero no me contestaste = I called you, but you did not answer. "Llamé" is the perfective past of "llamar" (to call) and "contestaste" is the perfective past of "contestar" (to answer), thus I am implying that I did it only one time of very few times. Te llamaba pero no me contestabas" = I called you but you did not answer. The translation provided for this example is literal, which means it translates word by word, but the real translation should be "I called you but you would not answer". In English you can use "would", even though it is a conditional auxiliary verb, it is used in past sentences to indicate that the action was performed many times. That is basically what you do with the imperfective forms in Spanish. You can imply that the action was extended in time or repeated many times during a given period of time. Other examples are: Yo no llegué = I did not arrive (perfective) Yo no llegaba= I would not arrive (over a period of time) (imperfective). Ella se cortó las uñas = she cut her fingernails (perfective) Ella se cortaba las uñas = she would cut her fingernails (every Saturday) (imperfective). I hope this helps you.