"Cậu bé uống nước ép."
Translation:The little boy drinks juice.
Vietnamese doesn't use definite articles, so an article not existing is the equivalent of a definite article in English. The word một can be used as an indefinite article- so if there was an article, it would me "a" because it means "one", but it not being present is the equivalent of "the" in English. When a noun doesn't have an article, it's showing definiteness, and thus the sentence is expressing "the". In English, we require articles to show specificity however in Vietnamese when you're not saying "one", you mean "the" in English. It's required for a grammatically correct English translation because English needs that specificity to show grammatical relation.