"Animalul vostru e gras."

Translation: Your animal is fat.

[LongHenry]

why is the definite article used here?

[potestasity]

In Romanian, you always use the definite article with possessive pronouns.
There is a way around it, but it isn't common in speech, it sounds more "poetic":
Al vostru animal e gras.

[LongHenry]

al meaning: of (your)? like 's

[potestasity]

Kind of. My knowledge of grammar fails me here; it is the same form as in this example:
The animal is yours. - Animalul e al vostru.
But I am pretty sure it serves a different function.

[HiMeCriss]

It isn't actually a pronoun ! It's an possesive adjective ! The possesive pronoun is: acest, această, acești, aceste etc. In current speech we use the possesive adjective, not the possesive pronoun ! This is also common in Greek and Bulgarian ;)

[TheLanses]

Vostru and tu is the same?

[Angi453279]

"Vostru" means "your"/ "yours" and it is used for plural. Depending of the sentence, it can be both possessive pronoun (al vostru) or possessive adjective.
"Tu" means "you". It is always used for singular, it is personal pronoun.
"Voi" means "you" for plural, personal pronoun.
"Tău" means "your"/ "yours", is for singular and it can be both possessive pronoun (al tău) or possessive adjective, depending of the sentence.