Most is a grammatically versatile word employed in references to amounts, quantities, and degree. This post discusses its use as various parts of speech.
Most, deriving from Old English and related to more, serves as an adjective pertaining to extent (as in “The most support comes from the Midwest”) or the majority (“Most of his supporters are in the Midwest”). Note the distinction between general and specific discussion: Compare “Most households have more than one television” (general) with “Most of the city’s households have more than one television” (specific). As an adjective suffix, it applies to something that most completely or extensively displays a characteristic, appearing in such words as foremost and hindmost, meaning, respectively, “farthest forward” and “farthest behind.”
As an adverb, most performs a similar function, except that it modifies adjectives. When it means “to the greatest degree,” it is preceded by the, as in “He found it to be the most rewarding job he had had to date.” When the meaning is “to a great degree,” the is omitted, as in “His current job is most rewarding.” It can also modify another adverb, as in the phrase “most certainly.” In addition, most is sometimes employed as a variant of almost to modify such words as all, anyone, anywhere, and always, as in “You will find that happens most everywhere,” but this usage is considered informal.
Most is also a noun meaning “the greatest amount,” as in sentences such as “It’s the most I can do” and “You gave him the most of all,” and in the phrases “at most” and “at the most” (which are interchangeable), as in “It will take her two or three days at most.” As a pronoun, it means “the greatest number or part,” as in “Most would agree.”