Okay môn is optional, but what is it for? Sports activities in general?
It's usually used for school subjects. It's hardly ever used for sports in everyday spoken Vietnamese.
So môn tiếng Anh would be like an English course?
I think the word for course was another (I hate that lesson and I cannot recall anything). Would be more like the English subject in school or maybe the English subject as part of a course or similar.
This is one of the first sentences that has really confused me grammatically. I have never seen a classifier used like this with a general activity (rather than referring to a specific ball, for instance) before.
Actually, the classifier "môn" is optional in these contexts.
In that case I revise my puzzlement to apply only to those cases in which the classifier is present, such as this sentence!
I wonder why this cannot translate as "She likes playing volleyball" as to me the meaning of "she like to play" and "she likes playing" are exactly the same.