"Ty dívky pijí vodu."
Překlad:The girls drink water.
The English definite article can easily be translated as nothing or as one of the Czech demonstrative pronouns (ten, ta, to, ti, ty, ta), depending on context, of which Duolingo provides absolutely none. Ty dívky pijí vodu can sometimes be a better translation of The girls drink water than Dívky pijí vodu.
Referring to your mention of "ta/ten = that, ty/ti = these", I think that you may want to look into the differences between THIS/THESE vs. THAT/THOSE a bit more.