The difference between the two words is that MAIN is an adjective and MAINLY is an adverb derived from MAIN. In other words, they pertain to different parts of speech.Thus, MAIN is exclusively used with nouns whereas MAINLY is used as an adverbial intensifier as illustrated below.e.g. the main road; the main idea; the main text; etc.e.g. (a) John mainly works as a legal advisor for Intel.(b) John lives mainly in NY.It might be more interesting if you phrase your question like this: What is the difference between MAIN and GOOD. I like to tell you that MAIN is one of the very few English adjectives that can only be used as a modifier as shown bellow.e.g. a) This is a main road.b) #This road is main.
