Learn Farsi: آیا هفتاد و یک صندلی در رستوران است؟ - Are there seventy-one chairs in the restaurant?

You combine the tens and the ones using the conjunction و (meaning "and"), which is pronounced "o" in this context. So, هفتاد (seventy) + و (o) + یک (one) becomes هفتاد و یک. The structure is exactly the same for all numbers from 21 to 99.

Why is the word for chair (صندلی) in the singular form if there are seventy-one of them?

In Farsi, whenever you use a specific number before a noun, the noun must stay singular. You never add a plural ending to a noun when counting it with a number. You literally say "seventy-one chair".

The English translation says "Are there", but the Farsi verb است means "is". Why is a singular verb used here?

There are two reasons! First, to say "there is" or "there are" to show something exists in a place, Farsi usually just uses است at the end of the sentence. Second, when the subject is a group of non-living things (like chairs), Farsi strongly prefers using a singular verb, even if there are many of them.

What is the purpose of the word آیا at the start of the sentence?

آیا is a formal question word that acts like a flag to show a "yes/no" question is coming. In everyday spoken Farsi, people usually drop آیا and just raise the pitch of their voice at the end of a regular sentence to make it a question.