Breakdown of El testigo declaró que no conocía al ladrón.
no
no, not
al
to the
conocer
to know (people, places)
que
that / than
Questions & Answers about El testigo declaró que no conocía al ladrón.
Why is conocía in the imperfect tense here?
This is a classic example of reported speech. The witness's original exact words at the time would have been in the present tense: I do not know the thief (no conozco al ladrón). Because we are reporting what he said in the past using declaró, the original present tense verb shifts a step back into the imperfect tense (conocía).
What if the sentence used the preterite conoció instead of conocía?
It would completely change the meaning! In Spanish, conocer in the imperfect means knew as an ongoing state, but in the preterite, it usually means met for the first time as a completed event. If the sentence said no conoció al ladrón, it would mean the witness testified that he did not meet the thief.
Why does it say al ladrón instead of just el ladrón?
This is a refresher on the personal a. Because the thief is a specific person receiving the action of the verb, we must put the preposition a in front of el ladrón. When a and el are next to each other, they always contract into al.