"Most of" is used to refer to a subset of a specifically stated group. In "Most of my students", this refers to a subset of students in the group "my students". Compare these phrases: "Most painters" - refers the majority of all painters "Most of the painters in this room" - refers to the majority of the painters in the group of painters in this room. Generally, "most of" is more specific. It specifically states the group you're talking about. Most people love this movie. Most of the people love this movie. You need an article. Most of them are right The fourth sentence in ungrammatical.