"Are the windows closed? It is raining!"

Translation: Az ablakok zárva vannak? Esik az eső!

[postkar]

why do I need 'lenni' in combination with zárva?

[jsiehler]

Zárva is an adverb; it has to modify a verb, and so the verb must be present.

(Also notice that zárva doesn't pluralize the way an adjective would in a nominative sentence like Az ablakok tiszták - "The windows are clean.")

[Bastette54]

This is interesting - from my English-speaking perspective, there's no difference in the parts of speech between "clean" and "open." Both are adjectives describing the state of the windows. I'm just talking about English here. Are you saying that in Hungarian, "zárva" is modifying "vannak" (in this sentence), rather than "ablakok"?

[jsiehler]

Yes, that's right. These can be called "adverbial participles", they're derived from verbs by means of the -va / -ve ending, they behave grammatically like adverbs and as such they modify verbs. Often the best English translation uses an -ing verb form, but when adverbial participles modify forms of "to be" they are often best expressed with adjectives, describing a state or condition, in English. Other than zárva and nyítva we don't have a lot of examples of verbs in this course that work well with van to express a state. But another one that's fairly common is megelégedik, which you will hear in its participle form in expressions like Meg vagyok elegedve, "I am satisfied."

For an example with a more active verb, A hegyről lenézve Pétert láttam = "Looking down from the hill I saw Péter."