Ok good even more words i can relate from russian xD Man the fact that i speak russian makes it a lot easier to learn polish (except the word "jest" bc that means "eats" in russian, got a small heart attack in the first lessons lmao ._.)
Verbs do not change case, they rather "cause" the case. Like the verb "pić" needs Accusative.
"mleko" and other neuter nouns have the Accusative form identical to the Nominative form. This also happens for masculine inanimate nouns and for 'not masculine-personal' plural.
Alternatively, to make it very clear that you mean "drinks" and not "is drinking", you could choose the habitual verb "pijać" and have "Ta kobieta pija mleko". But the 'basic' verb "pić" (pije) works perfectly fine and is a lot more common.