Based the lesson learnt, the formula is: imperfect cuando preterite.
What is the Spanish conjunction cuando about?
The conjunction cuando is one that is said to be accompanied by the preterite tense.
This due to the fact that it shows the specific moment an action occurred and of the verb that follows it.
The other verb that is seen in a sentence can be due to an ongoing action that is hindered by the preterite verb.
In such scenario, the ongoing verb is said to be in the imperfect tense as the case above.
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