Why dont use "ist" for is?
There's no present continuous in German.
so how i express ' a girl drinks.....' ????
It has the same translation as "A girl is drinking.", as there are no continuous tenses in German: "Ein Mädchen trinkt."
Are all nouns capitalized in German?
Simple answer: yes :)
Why ein is used here. Eine is feminine form right ?
Yes, but Mädchen is not feminine, it's neuter (though it may seem strange).
A girl drinking or is drinking...
"A girl drinking" is not a complete sentence. It must be "A girl is drinking" or "A girl drinks".
ein is for neuter words but madchen is feminine so why use ein instead of eine?
"Mädchen" is a neuter noun, though it may seem weird. The reason is that it's a diminutive (you can recognize them by the suffix "-chen"), which are all neuter.
Would, "a girl drinks" be the same translation as "a girl is drinking"?
Yes. German does not know progressive tenses. If you need to clarify that something is happening right now, you can add "gerade" (= at the moment) to the sentence ("Ein Mädchen trinkt gerade") - but it's not necessary.
A woman drinks
Nope, a "Mädchen" is a girl, not a woman ("Frau" in German).