Why some mansoubs have alif attached at the end and other don't?

[oceanic]

Salam

As i was going through exercise 3 on chapter 18 book 1. I noticed that words like kam ohtaan (how many sisters and also how many brothers) and kam qalamaan (how many pens) tend to have an extra alif attached to them at the end, when they become mansoub. But words like kam sayyaaratan (how many cars) don't.

Is there a reason to this?


Shukran

[hassan]

assalamu alaikum.

this is the way we write brother.
a noun with tanwin in the mansoob case takes an alif which is not pronounced. like:
كتابٌ كتابٍ كتاباً
but a noun ending in the round ta (ة) does not take this alif. like:
سيَّارةٌ سيَّارةٍ سيَّارةً

its mentioned in the Key, lesson 18, p. 49-50

[oceanic]

What is the reason then why in exercise 3 in ch18, question 8 kam bintan laka has an alif? (if taa marbuta does not take an alif).

Ahh just realised my mistake. Bintun has the normal ta not ta marbuta.