Typically, nouns are used with articles and adjectives. Nouns are also replaced by pronouns. When we use articles and adjectives to give more information about a noun, we create noun phrases. Nouns and noun phrases act the same when we use them in sentences.
noun (word) = students → they (pronoun)
noun (phrase) = the students (article + noun) → they (pronoun)
noun (phrase) = the smart students (article +adjective+noun) → they (pronoun)
Students can learn English very fast. → They can learn English very fast.
The students can learn English very fast. → They can learn English very fast.
The smart students can learn English very fast. → They can learn English very fast.
noun (word) = apartment → it (pronoun)
noun (phrase) = an apartment (article + noun) → it (pronoun)
noun (phrase) = a small apartment (article + adjective + noun) → it (pronoun)