Many times the differences between the preterite and the imperfect, which are the two simple past tenses of Spanish, are subtle enough that they aren't readily captured in translation. Thus "Mi vida fue feliz" (preterite) and "Mi vida era feliz" (imperfect) can both be translated to English as "My life was happy," and what little difference there is in the Spanish meaning can be picked up through the context. But sometimes, the difference between the two tenses is stark enough that they are translated differently in fact, the meaning may change enough that you can translate the verb with a different word.
For example, one difference between the two tenses is that while the imperfect typically refers to something that happened over an indefinite period of time, the preterite is sometimes used to indicate when the action of a verb began. A clear difference in translation can be seen with the verb conocer, which in the imperfect can be thought of as "knew." For example: "Conocíamos la cantante famosa" would usually be translated as "We knew the famous singer." But the preterite would refer to when that time of knowing began: "Conocimos la cantante famosa" means "We met the famous singer."
It works with the same with saber, the other verb for "to know," this time in the sense of knowing a fact or how to do something. In the preterite, can refer to learning or finding out something: Yo sabía que Alfonso era triste. I knew Alfonso was sad. Supe que Alfonso era triste. I found out Alfonso was sad.
Another distinction often made by the two tenses is that the preterite can be used to indicate that something actually happened, while the imperfect may not clearly indicate so. This is especially true where the imperfect indicates a state of being or status, thus making the preterite indicate an action of some sort. Note the following differences, where the imperfect is used first in each pair of examples:
- Yo quería ver la boda de mi tía. I wanted to see my aunt's wedding. (As in the English translation here, the Spanish sentence doesn't make clear whether the speaker saw the wedding.)
- Quise ver la boda de mi tía. I wanted to see my aunt's wedding (and I did).
- El café costaba diez pesos por taza. Coffee was priced at 10 pesos a cup.
- El café costó dies pesos por taza. The coffee cost 10 pesos a cup (for the person who bought it).
- Yo no quería comer los insectos. I didn't want to eat the insects. (It isn't clear what happened as a result.)
- No quise comer los insectos. I didn't want to eat the insects (and I didn't).
- Creían que era una buena idea que Antonio fuera el candidato del partido. They thought it was a good idea that Antonio be the party's candidate. (It's unclear whether he was the candidate.)
- Creyeron que fue una buena idea que Antonio fuera el candidate del partido. They thought it was a good idea that Antonio be the party's candidate (and he was).
- Podían nadar desde la playa a la isla. They could swim from the beach to the island. (It isn't clear whether they did.)
- Pudieron nadar desde la playa a la isla. They could swim from the beach to the island (and they did).
Similarly, use of the preterite can indicate that an obligation was met:
- Maricela tenía escribir un poema. Maricela had to write a poem. (We can't tell from this sentence whether she did.)
- Maricela tuvo que escribir un poema. Maricela had to write a poem (and she did).
- La ciudad necesitaba expandirse fuera de las murallas. The city needed to expand outside its walls. (We can't tell if it did.)
- La ciudad necesitó expandirse fuera de las murallas. The city needed to expand outside its walls (and did).