What is the difference between a most and the most?

Answer 1

There is definitely a difference between "He is a most unusual person;" and "He is the most unusual person I have ever met." "A most," while not that common in American English, implies that there may be others who are unusual, and he is simply one who exhibits this trait. Americans tend to use "very" in such a sentence-- he is a very unusual person; but in British English, you might still hear "a most". As for "the most," this is a definite statement, meaning there is no-one else who compares. This is the most fun I have ever had (there is no comparison-- it's the best, in other words).

Answer 2

"The most" is used to refer to the superlative form of an adjective or adverb when comparing more than two things. For example, "She is the most talented singer." "Most" alone is used as a determiner or adverb to indicate the superlative form of an adjective or adverb when comparing more than two things. For example, "She is most likely to win."