Breakdown of El año pasado encontré una telaraña muy grande.
una
a, an (feminine)
grande
big
el año
the year
muy
very
Questions & Answers about El año pasado encontré una telaraña muy grande.
Why is encontré in the preterite tense here instead of the imperfect?
Finding the cobweb was a single, completed event that happened at a specific point in the past (el año pasado). We use the preterite for completed past actions. You wouldn't use the lesson's hacía ... que + imperfect structure here because finding a specific web is a quick, one-time event, not a continuous or ongoing action.
I remember that encontrar has a stem change. Why isn't there one in encontré?
You are right that encontrar is an o-to-ue stem-changing verb in the present tense (for example, yo encuentro). However, regular -ar verbs do not have stem changes in the preterite tense, so it simply takes the standard 'yo' ending to become encontré.
How is the word telaraña (cobweb) formed?
Why does pasado come after año in el año pasado?
In Spanish, descriptive adjectives typically come after the noun they modify. Pasado describes which year it was, so it follows año. The whole phrase el año pasado is the standard set phrase for 'last year'.