What happens to Poder, Querer, Saber and Conocer in the Preterite?
If you understand the preterite as the tense that advances action, as the tense that reports an action as having happened or not, then this will come easily. The preterite is one of the two aspects of the past tense that is used to show the beginning or the end of an action in the past -- or the action as over and done. If that is clear, then the reason why these four verbs change meaning will come easily.
In otherwords, the preterite is used to express the perception of an action as finished: Comí la ensalada. (I ate the salad.) There is no salad left; this is done. It is also used if the onset or ending of an action is mentioned: Empecé a comer a las dos. (I satrted eating at two.) OR: Juan salió a las tres. (John left at three.)
Using this logic, if used in the preterite, the auxiliary verb PODER (indicating ability), must be focusing on one particular moment. Since the verb means to be able to, or can, then, if concentrated on a particular moment, it conveys the idea of either succeeding (it happened) or failing (it didn't happen): Juan pudo abrir la caja. (John succeeded in opening the box.) OR: Juan no pudo arreglar el carro. (John failed to/couldn't fix the car.) The idea of capacity is concentrated into a moment in which it has to happen or not.
The auxiliary verb QUERER (indicating desire), likewise focuses its energy on a moment in time in which either one wills to do something or wills not to. Hence, the affirmative of querer in the preterite answers to the English verb to try and the negative to refuse. Examples: Quise nadar una milla. (I tried to swim a mile.) OR María no quiso salir con Juan. (Mary refused to go out with John.) In addition to tried, the English verb attempted to is another situation in which the preterite of QUERER will be used.
SABER and CONOCER are unaffected by whether they are positive or negative. SABER, in keeping with its function of referring to learning about facts, means either to find out or to not find out. CONOCER, in keeping with its function of being famliiar with people, means to meet or to not not meet. Examples: Supe la verdad sobre tu hermana. (I found out the truth about your sister.) And: Conocí a Teresa anoche. (I met Teresa last night.)
When the primary meanings of these four verbs are needed: being able, wanting, knowing (information) or knowing (people), the imperfect must be used.