對於語言裡動詞沒有時態或語氣變化的我們而言，學習外語最大的困擾或是瓶頸常常是動詞變化、時態或語氣。我們必學的英語在這方面還是相對簡單的，起碼動詞變化沒那麼複雜。

西語裡相對於英文中的簡單過去式有兩種表現方法，過去式(el pretérito)與直述法未完成式 (imperfecto indicativo)。何時要用過去式？又何時應該用未完成式？實在令人困擾。每每在自以為了解後又會發現不同於自己歸納出來心得的例子。

在看過 Paralee Whitmire 在 SpanishDict.com 的課程講解後，自以為能掌握要領了，但好像又不太對勁。所以我想多看看別家怎麼解說，應該會有不少助益。



What's past is past, but in Spanish what's past may be either preterite or imperfect.

Unlike English, Spanish has two simple past tenses, known as the preterite (often called the preterit) and the imperfect indicative. (As in English, they are known as simple tenses to distinguish them from verb forms that use an auxiliary verb, such as "has left" in English and ha salido in Spanish.)

Although the English simple past in a sentence such as "he ate" can be conveyed in Spanish using either the preterite (comió) or the imperfect indicative (comía), the two tenses are not interchangeable. In general, the preterite is used when speaking of completed action, that is, when the verb refers to an action that has a clear end. On the other hand, the other past tense is known as an imperfect tense because "imperfect" can also mean "incomplete"; the imperfect indicative is used to refer to an action that doesn't have a specific ending. (At this point, it isn't important to know the meaning of "indicative," except that it contrasts with "subjunctive," a verb type that usually isn't studied by beginning students.) Following are some more specific uses that should clarify the difference; note that the imperfect is frequently translated in ways other than the English simple past.

The preterit is used:

To tell of something that happened once — Fui ayer a la tienda. (I went to the store yesterday.) Escribí la carta. (I wrote the letter.)

To tell of something that happened more than once but with a specific end — Fui ayer a la tienda seis veces. (I went to the store six times yesterday.) Leyó el libro cinco veces. (He read the book five times.)

To indicate the beginning or end of a process — Tuvo frío. (He got cold.) El huracán se terminó a las ocho. (The hurricane was over at 8.)

The imperfect indicative is used:

To tell of past habitual or repeated actions — Iba a la tienda. (I used to go to the store.) Leíamos los libros. (We would read the books.) Lavaban las manos. (They would wash their hands.) Escribía muchas cartas. (I wrote many letters.)

To describe a condition, mental state or state of being from the past — Había una casa aquí. (There used to be a house here.) Era estúpido. (He was stupid.) No te conocía. (I didn't know you.) Quería estar feliz. (He wanted to be happy.) Tenía frío. (He was cold.)

To describe an action that occurred over an unspecified time — Lavaban los manos. (They were washing their hands.) Cuando José tocaba el piano, María comía. (While José was playing the piano, María was eating.)

To indicate time or age in the past — Era la una de la tarde. (It was 1 p.m.) Tenía 43 años. (She was 43 years old.)

Other distinctions:

Background — The imperfect indicative is frequently used to provide the background for an event that is described using the preterite. Era [imperfect] la una de la tarde cuando comió [preterite]. (It was 1 p.m. when she ate.) Yo escribía [imperfect] cuando llegaste [preterite]. (I was writing when you arrived.)

Differences in translated meaning — Because of the way the two tenses are used, some verbs can be translated using differing words in English depending on the tense in Spanish. This is especially true when the preterite is used to indicate the beginning or end of a process. Conocí al presidente. (I met the president.) Conocía al presidente. (I knew the president.) Tuvo frío. (He got cold.) Tenía frío. (He was cold.) Supe escuchar. (I found out how to listen.) Sabía escuchar. (I knew how to listen.)

Final note:

Some of the sentences on this page could be stated in the other tense with a change of meaning. For example, while "escribía muchas cartas" would be the typical way of saying "I wrote many letters," as that is something that typically would take place over an unspecified period of time, one also might say "escribí muchas cartas." But the meaning of the sentence, not readily translatable without a context to English, would change to indicate that the speaker was referring to a specific point in time. For example, if you were talking about writing many letters while you were on a particular trip, you might use the preterite form.

Conjugations for the preterite and imperfect are explained in separate lessons.

http://spanish.about.com/od/verbtenses/a/two_past_tenses.htm

Imperfect: Part I

The imperfect is used for past actions that are not seen as completed. Use of the imperfect tense implies that the past action did not have a definite beginning or a definite end. You also learned how to conjugate regular -ar verbs. In this lesson, you will learn how to conjugate -er and -ir verbs, and become more familiar with the uses of the imperfect.To conjugate regular -ar verbs in the imperfect, simply drop the ending (-ar) and add one of the following:

aba
abas
aba
ábamos
abais
aban

To conjugate regular -er and -ir verbs in the imperfect, simply drop the ending (-er or -ir) and add one of the following:

ía
ías
ía
íamos
íais
ían

Here are all three regular imperfect verb forms together:

hablar	comer	vivir
hablaba	comía	vivía
hablabas	comías	vivías
hablaba	comía	vivía
hablábamos	comíamos	vivíamos
hablabais	comíais	vivíais
hablaban	comían	vivían

　The imperfect is used for actions that were repeated habitually.

Almorzábamos juntos todos los días.
We would lunch together every day.

Las señoras siempre charlaban por las mañanas.
The ladies would always chat in the mornings.

　The imperfect is used for actions that "set the stage" for another action.

Yo leía cuando entró mi papá.
I was reading when my papa entered. (note that "entered" is preterite)

　The imperfect is used for telling time and stating one's age.

Eran las siete de la noche.
It was seven o'clock at night.

La niña tenía cinco años.
The little girl was five years old.

The above examples all fall within our general rule for using the imperfect:

The imperfect is used for past actions that are not seen as completed.

http://www.studyspanish.com/lessons/imp1.htm

Imperfect: Part II

Remember, the imperfect is used for past actions that are not seen as completed. Use of the imperfect tense implies that the past action did not have a definite beginning or a definite end. The imperfect tells when -- in general, an action occurred.

Good news! There are only three irregular verbs in the imperfect. You must simply memorize them.

ser	ir	ver
era	iba	veía
eras	ibas	veías
era	iba	veía
éramos	íbamos	veíamos
erais	ibais	veíais
eran	iban	veían

To review some of the rules for using the imperfect:

• The imperfect is used for actions that were repeated habitually.
• The imperfect is used for actions that "set the stage" for another action.
• The imperfect is used for telling time and stating one's age.

 http://www.studyspanish.com/lessons/imp2.htm