Why does he sometimes prounouce eserah and some esereh, can I use both, notice that I'm not asking about the genders matter. It's just about the pronounce of the masculine form's עשר only.
In this lesson, if the numbers are used without the context, we are supposed to be using female forms. Why are male forms being used here?
There are two different things to distinguish. If a number counts something, than the number should be masculine or feminine. The something can be said explicitly in the sentence, or implicit from the context. Here they give an example of talking about the number of something masculine that is known in context.
Then, there are cases of using numbers as abstract, not counting something in particular. Then the feminine form is used.