Most high schoolers encounter at minimum three verb tenses in Spanish: Present, Preterite, and Imperfect.
Present tense is, of course, in reference to actions taking place in the present. Preterite and imperfect are both what an English speaker would call past tense, however there are distinctions in both their use and conjugation. Preterite tense is used in reference to actions that took place in the past, but were a "one time thing", whereas imperfect is used to refer to actions that are repetitive or habitual.
This concept can be very confusing at first, because English does not make these distinctions when speaking about the past. Students are often frustrated with having to determine which tense is appropriate to use, and then determine for which pronoun they should conjugate the verb.
Preterite: I went to school yesterday. (Yo fui a la escuela ayer.)
Imperfect: I went to school everyday last year. (Yo iba a la escuela cada dia del año pasado.)
In the above example, we can see that preterite is used to describe going to school in one instance, whereas imperfect is used to describe going to school repetitively over a period of time. This example illustrates the distinction between the uses of preterite and imperfect when speaking in past tense.
There are some key words that can aid us in determining when to use imperfect or preterite tense. For example, words or phrases like "always", "everyday", or "when I was a child" indicate repetition and are therefore indicative of imperfect. Words like "yesterday", "earlier", or "last week" are most often associated with the use of preterite.
Some students also find it difficult to remember which tense is which (i.e. "I know one is repetitive and one is for things that happened once, but I can't remember which is which"). In this case, remind them that "practice makes (im)perfect"! In other words, if you do something imperfectly, you must practice, practice, PRACTICE! Remind them that the repetitiveness of practicing something over and over and over is an example of when you would use the imperfect tense!