The preterite and imperfect tenses are often used together in the same sentence. There are a few typical sentence‐structure formulas that are easy to learn. Two vocabulary words that are necessary to join two past tense verbs in the same sentence are mientras (while) and cuando (when). Notice that cuando does not have an accent mark because it is not being used as a question word.
Joining verbs with mientras
The conjunction mientras can be used to join two verbs in the imperfect, or it can be used in sentences where one verb is in the preterite and the other is in the imperfect. The sentence will change meaning depending on the tense of the verbs joined by mientras. Read the explanations carefully and then analyze the examples after each formula. Visual cues are provided in both the formula and example: A verb in the imperfect tense is underlined, and a verb in the preterite is in bold.
To indicate that two actions are occurring simultaneously and are assumed to be ongoing, the conjunction word mientras is used between two verbs, and both verbs will be conjugated in the imperfect tense. It is necessary that neither action interrupt the other for both verbs to be conjugated in the imperfect tense.
Sentence structure formula: Imperfectmientrasimperfect
Ellos secaban los platos mientras yo los lavaba.
They were drying the dishes while I was washing them.
When the verb following mientras is in the imperfect and the other verb is in the preterite, the action of the verb in the preterite interrupts the ongoing action of the verb in the imperfect.
Sentence structure formula: Preteritemientrasimperfect
Samuel llamó mientras tú estudiabas.
Samuel called while you were studying.
Both in Spanish and English, the previous sample sentence could be switched around. It is acceptable to have mientras in between the two verbs, or mientras may begin the sentence followed by a verb in the imperfect, a comma, and a verb in the preterite. Regardless of the order of the sentence, the ongoing action will be in the imperfect, and the interrupting verb will be in the preterite. This is a very common sentence structure in the past. For example, when mientras and the ongoing action is stated first, there will be a comma before the interrupting verb (in the preterite):
Sentence structure formula: Mientrasimperfect, preterite
Mientras tú estudiabas, Samuel llamó.
While you were studying, Samuel called.
Joining verbs with cuando
The conjunction cuando is almost always followed by the preterite tense because it indicates the specific moment of the action of the verb that follows it. The other verb in the sentence may be an ongoing action that is interrupted by the preterite verb. In such cases, the ongoing verb is in the imperfect tense.
Sentence structure formula: Imperfectcuandopreterite
Alicia trabajaba cuando yo entré en la oficina.
Alicia was working when I entered the office.
Just like sentences using mientras, the sentence order can be switched around, but the verb following cuando will be conjugated in the preterite tense, regardless of where it is placed in the sentence.
Sentence structure formula: Cuandopreterite, imperfect
Cuando yo entré en la oficina, Alicia trabajaba.
When I entered the office, Alicia was working.
As you can see in the previous example, when cuando is used at the beginning of the sentence, it will be followed by the preterite and a comma. After the comma, the next verb will be in the imperfect.
In some sentences, the conjunction cuando is used to join two preterite verbs. This sentence structure indicates that the action of both verbs was completed at the same time or that one was the result of the other.
Sentence structure formula: Preteritecuandopreterite
Carlos me llamó cuando él llegó.
Carlos called me when he arrived.
Yo grité cuando vi al fantasma.
I screamed when I saw the ghost.
Sometimes the same basic construction is reversed. Whenever the conjunction cuando is the first word of the sentence, a comma is placed between the two preterite verbs.
Sentence structure formula: Cuandopreterite,preterite
Cuando él llegó, Carlos me llamó
When he arrived, Carlos called me.
Cuando vi al fantasma, yo grité.
When I saw the ghost, I screamed.