Arabic has specific grammar rules for ‘Aadad and Ma’duwd–the number and the counted.
Note: These rules apply for numbers from three to ten. One and two are special-they count as adjectives, not numbers, because the form of the word implies the number.
In English, we say “three cars”. Three is the number (‘adad), and cars is the counted (ma’duwd).
What are the grammatical rules of ‘adad and ma’duwd?
Similar to time modifiers, ‘adad and ma’duwd work like possessive case. The ‘adad always comes first, and it behaves like the possessor–so it’s definite. The ma’duwd comes next, and it behaves like the possessed–so it takes kasra.
The other rule is that ‘adad and ma’duwd have opposite genders–so one will be masculine and one will be feminine. Which determines the other? The ma’duwd (counted thing) determines the gender of the ‘adad (number).
One implicit rule is that the ma’duwd is always plural–which is true because we’re counting more then two. (In Arabic, we have singular, dual, and then plural–so two is not plural.)
So say we wanted to say “five students”. Students is “tullaabun” (masculine), five is “khamsa“.
So our first guess might be “khamsa tullaabun”.
But then, we realize tullaabun is masculine, so we feminize “five” by adding ta-marbuwta. Our next guess might be “khamsatun tullaabun”.
Then we remember that they act like the possessive case–so student takes kasra. The final (correct) version is “khamsatun tullaabin“.
What about “three cows”? Three is thalathun, cows is baqaraat–so we get “thalathun baqaraatin“.
Four pens? Arba’atun aqlaamin! Again, it’s Arba’atun because aqlaam (pens) is masculine.
Seven spoons? Sab’aa mil’aqatin! Sab’a, no sab’ah (with ta-marbuwta), because mil’aqah (spoons) is feminine.
Inshallah you can find some of these in the Qur’an and post them in the comments–the ‘adad, ma’duwd, and the verse they occur in.