Most and Mostly
The words came up in a discussion recently. They look so similar that it seems like they would follow the pattern of other adjectives and adverbs.
quick (adj.), quickly (adv.)
painful (adj.), painfully (adv.)
slow (adj.), slowly (adv.)
With these words, the adjectives have a meaning that is very similar to the adverb. Thus quick and quickly both mean in a rapid way. Painful and painfully both describe strong or severe pain while slow and slowly synonymously refer to doing something at a low speed.
Most and mostly don't follow that pattern. They are a little like another pair, hard and hardly.
Most is an adjective, but it usually is used with another adjective to show the superlative degree, that is, the highest or only one of something as in this sentence:
He is my most trusted friend.
This means that I trust no friend more than him.
Now when I change most to mostly, there is a change in meaning.
He is my mostly trusted friend.
Mostly here means that I usually trust him, but it also implies that he is not my most trusted friend because I can't trust him all of the time.
Most also behaves a little like an adverb because it is usually found with an adjective next to it. It modifies the adjective. But dictionaries classify it as an adjective. Also, most is usually preceded by the article, the, unless there is a possessive, which is even more definite than the article.
Hard and Hardly
When I was young, I would often hear one person trying to be a little funny asking another person,
Are you working hard or hardly working?
Hard and hardly mean the opposite, thus the use of or. Working hard means a person is doing the job to the best of his or her physical or mental powers. Hardly working means the person is doing as little as possible. This contrast is much stronger than most and mostly.
Of the other superlatives, least, worst, and best, there is no adverb (-ly) form, so they have nothing to be confused with like most and mostly.