Why is "אתה מספר אחד עשר" not acceptable?
Because ordinal numbers are always in the female. יש פה שנים עשר אנשים ואתה מספר אחת עשרה. It's not a number of people, it's an ordinal number.
Why is eleven necessarily ordinal in this phrase? E.g. in context of basketball game wouldn't it be considered cardinal?
I am sure Almog meant to say "ordinary" instead of "ordinal". This is not an ordinal number.
But we'll never know for sure, as Almog hasn't been active for a very long time.
I meant, שנים עשר אנשים counts the people so is in the male form. מספר אחת עשרה is ordinal.
So after עשירי ordinal numbers don't have the yud or yud-tav in the end and sound like cardinal numbers?
If I remember correctly, ordinal and cardinals are the same for numbers 11 and up. The numbers 1-10 have different ordinal forms. Particularly the number 1. I assume the course will go into it in another lesson.
Ata mispar akhat esre.
Seems like attaching a number to a male would make the number masculine... but I'm probably thinking too much into it.
That would work if numbers were adjectives. But they are not. The number here is abstract, so it is feminine by default.