Learn Farsi: آیا شلواری و جورابی روی تخت هستند؟ - Are a pair of pants and a sock on the bed?

This -i is the indefinite marker in Farsi. It works just like the English words "a" or "an" to show you are talking about any general item, not a specific one. By adding it, shalvar becomes shalvari (a pair of pants) and jurab becomes jurabi (a sock).

Since "pants" is plural in English, why does shalvar take the singular indefinite marker -i?

Unlike English, Farsi treats clothing items like pants or glasses as standard singular nouns. Shalvar simply means one piece of clothing (one pair of pants). That is why it can take the singular indefinite suffix -i just like a regular object would.

If shalvari and jurabi are both singular items, why is the verb hastand (are) in the plural form?

Even though each item has a singular indefinite marker, they are joined by the word va (and). Because the total subject (a pair of pants AND a sock) adds up to more than one thing, the verb at the end of the sentence must be plural.

Is the word aya at the beginning required to make this a question?

No, aya is an optional question word that makes the sentence formal and clear. In everyday spoken Farsi, you can just drop it entirely and raise the pitch of your voice at the end of the sentence to show you are asking a question.