"Bananas are fruit."
Translation:Chuối là trái cây.
You use the plural modifier to talk about actual objects. In this case bananas describes bananas in general. In general you might say "chuối có màu vàng". But you might have some specific bananas which turned brown, so you describe them like this: "những trái chuối này có màu nâu". Similarly you only need the classifiers such as "trái" or "quả" when you talk about specific objects or to clarify that you mean "chuối" the fruit. Some words have multiple meanings. This is especially useful as a foreigner trying to speak Vietnamese as it narrows down what you mean when you can't properly pronounce the different tones. In this case however it is already clear you talk about the fruit, so you do not need a classifier before "chuối".