'Conocí', 'supe', 'no pude': verbs that change meaning in the preterite
Some very common Spanish verbs describe states: knowing, being able, wanting, having. In the imperfect they describe that state as ongoing background. Put them in the preterite, though, and the focus shifts to a single moment: the point where the state began, or the result of trying. So conocía a María means I knew María, but conocí a María means I met her. Sabía la verdad is I knew the truth, while supe la verdad is I found it out.
English speakers trip over this because English uses one past form, knew, for both, and then needs a different verb, such as met, found out, managed or refused, to express the event. If you translate word by word, you will either miss these meanings when you read or sound odd when you speak. The main verbs to watch are conocer, saber, poder, querer, tener and costar.
The preterite presents an action as complete, with a clear start or end. A verb that describes a state has no natural action inside it, so the preterite picks out the edge of the state: the moment it started (met, found out) or how an attempt turned out (managed, failed). The imperfect simply describes the state as it was.
Rule of thumb: when you see one of these verbs in the preterite, ask yourself what happened at that moment? The English translation is usually an action verb, not knew, could or wanted.
This is a tendency driven by context, not an automatic dictionary switch. The preterite gives you an action meaning, and the rest of the sentence confirms which event is meant.
With conocer, the preterite marks the first encounter: meeting someone, or visiting a place for the first time.
With saber, the preterite is the moment the knowledge arrived, so it usually means found out or learned.
With poder, the preterite reports the result. Affirmative pude means you managed to do it. Negative no pude means you tried and failed.
With querer, the affirmative preterite often suggests that the wish turned into an attempt. The negative no quiso very often means someone refused.
With tener que, the preterite says the obligation was real and you carried it out. The imperfect only says it existed, and leaves open whether you actually did it.
With costar in the sense of being hard, the preterite looks back on a finished effort. It usually implies you got there in the end.
Do not treat these meanings as fixed translations. Quiso does not always mean tried. It can simply report a wish that arose at a specific moment. Likewise, tuvo does not automatically mean received. Tener gets an action meaning only in clear situations with a start and end, such as tuve una idea (I had an idea, meaning it came to me) or the tuve que pattern above. Let the context decide.
The typical learner mistake runs the other way: using the preterite to describe background knowledge. Supe que era médico means you found out he was a doctor. To say you simply knew it, use sabía. Likewise, for someone you knew for years, use conocía, not conocí.
Also remember that negation changes the picture for poder and querer. No pude is a failure and no quise is a refusal, both definite outcomes. The imperfect versions no podía and no quería only describe an inability or an unwillingness, without saying what happened.
With conocer, saber, poder, querer, tener que and costar, use the imperfect to describe how things were and the preterite to report what happened. In the preterite, think of an event in English: conocí = I met, supe = I found out, pude = I managed, no pude = I failed, no quise = I refused, tuve que = I had to and did.