Ser or Estar? Preterite or Imperfect?
Deciding between ser and estar is a daunting task for most beginning students of Spanish. The problem becomes compounded once they learn the preterite and the imperfect. Not only are both of these verbs irregular in these two past tenses, they are irregular in the present as well. Therefore, the morphological features of these verbs add even more fuel to the fire of confusion.
First, here is a very useful rule of thumb for deciding between them when they are not being used with adjectives or as helping verbs:
The verb estar is used for health and location while ser is used for everything else. What is meant by everything else? From a practical point of view, this rule stands on its own as a default setting. It is highly reliable: If one needs a form of one of the two verbs to be in Spanish, he or she needs only to ask if the situation is one of health or location. If it is about either of these, then the speaker only needs to conjugate the verb so as to agree with the subject in person and number.
Let's review the forms of ser and estar in the present indicative:
Next, in the preterite:
Finally, in the imperfect:
The forms of all tenses of these verbs are best dealt with by memorizing them -- and knowing which is which. But the usage question still remains, and now, for any given person and number, when going from English to Spanish, there are four possibilities. All of them translate into English as either was or were -- so there are twenty-four ways to say was or were in Spanish! Clearly, the decision algorithm or tree has to be pruned a lot.
My suggestion for cutting this problem down to size may be somewhat controversial, but I have spoken with people from all over the Spanish-speaking world who have admitted, after some initial discomfort (depending on where they are from), that my suggestion is feasible. Even those who have been least convinced by my argument have conceded that it is a good rule for beginners -- another one of my "90%" rules:
Don't use the preterite of estar! Why? Because although there are good reasons for using it, such as when a time of day is mentioned in conjunction with where someone was or wasn't (the preterite must be used -- it falls in the other 10%!), there are very few other situations in which the imperfect of estar could be used with no distortion in meaning.
By eliminating the preterite of estar from possible solutions, the decision algorithm goes as follows:
1. Does the situation involve health or location in the past? If so, the imperfect of estar must be used and the only remaining task is the person and number.
2. If the situation does not involve health or location in the past but the use of the be verb falls in the "everything else" category, then there is one last question to ask: Is one summarizing or introducing an idea? The preterite, remember, sees an action as over -- it has a summarizing quality. The imperfect has an on-going quality, and is descriptive and open ended.
Example: Consider a question like ¿Quién fue Simón Bolívar? -- Imagine a child standing in front of a parent and asking the question. The answer Fue un general pretty much closes the subject. It's time for dinner and there is no time for elaboration -- book closed. On the other hand, Era un general suggests strongly that the parent is about to take the time, open up the book, so to speak and begin to tell the child more.