Native Korean numbers are used to express time and unit. They are generally used along with unit nouns that express the appropriate unit when counting things or numbers. 하나 is a number (noun) when counting something. And 한 is an adjective. Therefore, use 한 with a noun. Eg. 연필 하나 (one pencil) 연필 한 개 (one pencil (with a counting particle 개))