Learn Polish: Spróbuj odblokować te drzwi.

In this lesson's context, od- acts as a reversal prefix. The base verb is blokować (to block or to lock). By adding od-, it creates odblokować, which means to reverse that state: to unblock or to unlock.

Why does the sentence use the plural pronoun te for a single door?

The Polish word for door, drzwi, is a "plurale tantum" noun, meaning it only exists in the plural form. Even if you are talking about just one door, you must treat it grammatically as a plural noun, which is why we use the plural demonstrative pronoun te (literally "these").

Why are both verbs in this sentence perfective (Spróbuj and odblokować)?

You use the perfective imperative Spróbuj because you are asking someone to make a single, completed attempt. You follow it with the perfective infinitive odblokować because the goal is the completed action of the door being successfully unlocked, rather than just the ongoing process of fiddling with the lock.

What case is te drzwi in this sentence?

It is in the accusative case, acting as the direct object of the verb odblokować. Because drzwi is a non-masculine-personal plural noun, its accusative form looks exactly the same as its dictionary (nominative) form.