"Your children are big."
Translation:Copiii tăi sunt mari.
Copiii with three "i" means that is plural and have the article the, but If you put tău the article disapear. The other "i" means tău? Someone can explain me?
I'm not sure I understand what you're referring to. "Tău" is the possessive adjective for a single possessed object, so "copilul tău" means "your child". "Copiii tău" or "copii tău" or "copii tăi" are not valid constructs, if you encountered one of them, it was a course error.