Why does 'mleko' not change in a similar manner as 'woda' is changed to 'wodę'?
because it is neuter noun, and neuter nouns have accusative=nominative,
Please ask me if you do not know what cases are.
Got it, thanks!
It said jestem, not ja.
It is because English is complicated. "I am" = (ja) jestem , "I am drinking"=(ja) piję, I drink= (ja) piję.
Polish - "ja" =I is not necessary, because you can tell from verb form.
English - "am drinking" is a verb form for "to drink" 1st person present continuous.