For abstract nouns, or nouns used in an abstract sense, you drop the article. So there's a slight difference in meaning: Natur ist schön = abstract nature in general is beautiful Die Natur ist schön = nature thought of more concretely, as some particular thing, is beautiful. It doesn't work with nouns that are not used abstractly: "Ich bin in Schule" is wrong. But you could say both "Schule ist nützlich" and "Die Schule ist nützlich".