"No, I am drinking water."
Translation:Nie, piję wodę.
I wanted to write a reply, but Duolingo has this covered here: https://www.duolingo.com/skill/pl/Basics-2
In Polish, a sentence where the pronoun has been omitted is still perfectly grammatical.
When talking about yourself, you usually skip the ja pronoun, unless you want to emphasize something. The same goes for all 1st and 2nd person pronouns.
This is because of the way conjugation works. The verb forms used with 1st and 2nd person pronouns are unique for these pronouns. Even if you omit the pronoun, the verb still reveals who is the person performing the action, so there is no need to mention it explicitly.
However, omitting 3rd person pronouns may not always be appropriate. Generally, you only skip them if the context makes it clear who you are referring to.
As there are separate 3rd person pronouns for different genders, they contain information about the gender of the subject that cannot be in any way deduced from the form of the verb.
Consequently, 3rd person pronouns cannot be ommited in sentences where the gender of the pronoun is used to differentiate between subjects, such as On je jabłko, a ona je chleb (He is eating an apple and she is eating bread).
I'd add that omitting 3rd person is more often used in past and future tenses where you can deduce gender from a verb form, and that in the last example also, other pronouns should not be skipped (she eats an apple I eat bread- Ona je jabłko, ja (jem) chleb.
That is an equivalent of "No I am I drink water". It makes exactly zero sense.
Don't think of the Present Continuous construction ('to be verb-ing') as something that actually contains any real 'to be'. It doesn't. It's just a construction. It's just one verb. You cannot translate 'to be'.