copil = child (sing.) --- copii = children (plural) (without any articles)
un copil = a child --- niște copii = some children (indefinite article)
copilul = the child --- copiii = the children (definite article)
Instead of appearing before the noun, the definite article is added to the end.
băiat = boy (sing.) --- băieți = boys (plural) (without any articles)
un băiat = a boy --- niște băieți = some boys (indefinite article)
băiatul = the boy --- băieții = the boys (definite article)
There are two different words with two different meanings.