Most refers to the greatest amount, degree, or a majority of a group, as in most of the guests had already left, while mostly means mainly or generally, describing how something is largely the case, as in the audience was mostly students. Think of most as counting a quantity and mostly as describing a general pattern.
The core difference
Most measures amount or majority, while mostly describes something as being generally or mainly the case.
- most — adjective/pronoun, greatest amount or majority: Most of the guests had already left by midnight.
- mostly — adverb, mainly or generally: The audience was mostly students.
- most — adverb forming a superlative: That was the most exciting match of the season.
How to tell them apart
Most has three jobs: it can describe an amount (the most sugar), stand in for a majority (most of the guests), or build a superlative before a longer adjective (the most exciting match). Mostly has one job, describing something as largely or generally true, as in it rained mostly in the afternoons that week.
A useful check is to try swapping in mainly: if mainly fits the sentence, mostly is the word you want. If you are instead counting or comparing an amount, most is correct.
