1.  Most = adjective

(adjective + noun)

Ex:  Most students spend a lot of time doing their homework.

THIS IS THE MOST COMMON USE OF “MOST.”

2.  Most of = pronoun, preposition

(pronoun + preposition + pronoun)

Ex:  Most of them also have to work.

YOU MUST USE AN OBJECT PRONOUN AFTER THE PREPOSITION.  NO EXCEPTIONS!  (Most of + noun DOES NOT EXIST!)

Ex:  Most enjoy studying.  (In this sentence, “most” is also a pronoun.  It is understood that I am referring back to the subject “students” mentioned in a previous sentence.)

3.  Most of the = pronoun, preposition, article

(pronoun + preposition + article + noun)

Ex:  Most of the students that I know live with their families.

IN THIS USAGE, YOU ARE REFERRING TO A NOUN (“students”) THAT YOU FURTHER DEFINE ("that I know") or that you've mentioned previously.

In order to understand the difference between the three, you must understand that prepositions like “of” must always be followed by either a noun or a noun phrase (adj + N).  You must also know that the object of a preposition can NEVER be the subject of a clause.  Therefore, in the second and third examples, “most” is a pronoun (it is the subject of the sentence, taking the place of a noun), but in the first example it is an adjective because it immediately precedes a noun.  You must also know that “the” is necessary before the noun (as in the third example) because, when you use “most,” you are referring to a part of a specific group (“students that I know”).