When forming numbers from 13 to 19, the number consists of a unit (3-9) followed by the word for ten. Just like with other numbers from 11 to 99, the noun being counted must always be singular and accusative.
The Reverse Agreement Rule
For numbers 13 through 19, the two parts of the number follow completely different gender agreement rules:
- The unit (3-9) takes the opposite gender of the noun.
- The ten (10) matches the gender of the noun.
When you count a feminine noun, the unit drops the taa marbuta (taking a masculine form, like ثلاث), while the ten keeps the taa marbuta (taking a feminine form, عشرة).
Counting from 15 to 19
The same rule applies all the way up to 19. Notice how the units (خمس, ست, سبع, ثماني, تسع) are all in their masculine forms to contrast with the feminine nouns they are counting (such as شجرة or زهرة), while عشرة remains feminine.
Comparing Masculine and Feminine
To see this reverse agreement rule in action, look at what happens when we count both a masculine noun (نوع, type) and a feminine noun (نبتة, plant) in the same sentence.
For the masculine noun, the unit gets the taa marbuta (أربعة) and the ten drops it (عشر). For the feminine noun, it is exactly the reverse!
