"I am drinking her water."
Translation:Ich trinke ihr Wasser.
Umm, "ihr" is the Possessivartikel for her -although it is for your (formal third person) too.
The possessive for "your" (formal) would be Ihr -- capitalised.
ihr (lowercase) can mean "her" or "their" as a possessive determiner.
Because Wasser is neuter but ihre is used before feminine or plural nouns.
Because it isn't.
The possessive adjectives mein, dein, sein, ihr, unser, euer, ihr, Ihr inflect like ein and kein, and have no ending in the masculine nominative, neuter nominative, or neuter accusative.
I don't know why. There probably is no answer to "why".
The possessive pronouns do have an ending, though, just as the indefinite pronoun (or whatever it's called) does: dein Wasser ist blau aber ihres ist grün "your water is blue but hers is green" (note that English, too, has an -s on "hers" which is not present in "her water"); ich habe ein Pferd und du hast auch ein(e)s "I have a horse and you have one, too".
In simple terms, accusative case refers to the direct object of a verb, i.e. the thing that is directly acted on. Dative is the indirect object of a verb (often, but not always, indicated by the use of a preposition before it), i.e the thing that is indirectly affected by the action of the verb. e.g. In "The boy hit the ball to the girl" the ball is the direct object that is directly acted on (and therefore accusative case) and the girl is the indirect object that receives the thing acted on (and therefore dative case).
Note that some English verbs that take accusative objects are translated into German verbs that take dative objects. These are often referred to as German dative verbs because their objects are always dative case rather than the more normal accusative case.
No, because Wasser is neuter, not masculine.
If you had a masculine noun such as Wein, it would indeed be ihren Wein.
But for non-masculine words, the accusative looks just like the nominative, e.g. ihr Wasser, ihre Milch, ihre Getränke.