What is the difference between the preterite and the imperfect tense?

Firstly, I will explain the theoretical concept of both tenses to the student. In this case, both tenses place the action in the past, but the meaning of both tenses is radically different. On the one hand, the preterite tense emphasizes the action in the past. An example is:'Yo viajé a España el verano pasado', which means that I traveled to Spain last summer. The important part of the sentence is that I traveled.
On the other hand, the imperfect sense emphasizes the duration of the past action. An example is 'Yo viajaba a España el verano pasado, cuando conocí a mi amigo Juan', which means that I was traveling to Spain last summer when I met my friend Juan. The important part of the sentence is that I met my friend Juan (conocí is in preterite tense) and the part 'was traveling (viajaba is in imperfect tense)' refers to the underlying action.
In some sense, the student can understand the difference between these two tenses as the difference between the past simple and past continuous. However, we need to practice the conjugation of verbs, do some exercises in order to present different cases to the student and study the peculiar case of irregular and reflexive verbs.