"The girls are drinking water."
Translation:Les filles boivent de l'eau.
Partitive articles ("du", "de la", "de l' " and "des") are explained here:
In French the nouns have genders. It is best to try to remember which are which. You know this by le and la (meaning the). A word that has le is masculine and a word that has la is feminine. Le tigre is masculine. La lettre is feminine.
L'eau means "(the) water". You wouldnt use "d'eau", as french nouns need an article. However, "de l'eau" would be correct, which means "some water"
However, albeit "eau" being masculine, you don't use du. To say "some water", you would use "de l'eau". You only use "du" when the "le" is fully present. Because of the Deux Voyelles Ensemble rule, "Le eau" becomes "L'eau," so you use "De l'eau" rather than "D'eau". To help remember this, keep the article with the word and add "de" beforehand. If it reads "de le ...", then make it "du" and if it reads "De l'...", then leave it be.
In French if a specific amount or quantity isn't given you have to say "some" in the translation. In English they are drinking milk, in French they are drinking some milk. You don't have to say some if you say they are drinking a glass of milk.
"Elles" refers to a group of females, and is more a "they" meaning. The sentence was specifically about "the girls", so "elles" isn't quite right.