Learn Arabic (Modern Standard): هناك واحد وعشرون عقارا معروضا في السوق. - There are twenty-one properties listed in the market.

In compound numbers ending in 1 or 2 (like 21, 32, 51), the 'one' or 'two' part must match the gender of the counted noun. Since the counted noun عقارا (property) is masculine, we use the masculine واحد.

Why is 'twenty' written as عشرون instead of عشرين?

Because 'twenty-one' is the grammatical subject of this sentence (it is the thing that exists after هناك). Subjects take the nominative case. For multiples of ten, the nominative case uses the ـون ending, while ـين is used when the number is the object of a verb or follows a preposition.

We are talking about multiple properties, so why is the word عقارا singular?

This is a strict rule in Arabic: numbers from 11 to 99 must be followed by a singular noun in the accusative case. The extra alif at the end of عقارا shows this accusative ending. So, you are literally saying 'twenty-one property.'

Why does the adjective معروضا (listed) also end in an extra alif?

Adjectives must copy the grammatical case of the noun they describe. Because the number 21 forced عقارا into the accusative case (adding the alif), its adjective معروضا has to follow suit and take the accusative case as well.