When should you use the preterite tense?

It depends on the context and sentence. But, as a general rule, you use the preterite when you're talking about an action that had a clear start and a clear end. An example of that would be 'Paul taught his student for 3hrs'. The preterite works here because you can picture the clear time limits. Or, 'I ate a salad for lunch'. Here, you can pinpoint when this action happened and when it ended. I'll give you one last, slightly tricker, example, 'I bought a new car'. Even though you have no idea when I bought my car, it is obvious that the action of buying had an end. We're talking about a completed action, so we use the preterite.