1. That's fine with me.

(Does 'with' mean 'being related to'?)

2. It's raining much.
3. It's raining a lot.
4. It's raining hard.
5. It's raining heavily.
6. It's raining cats and dogs.
(Are they all the same in meaning? Can we use 'much' instead of 'a lot'?)

#1. I don't think that's a good explanation. It just means that I agree.

#2 nope - However, it's common to say "It's not raining much"

1. In the sentence "That's fine with me," the word "with" does not mean "being related to." In this context, "with me" is an idiomatic expression that means "I am okay with it" or "I agree." So, "with me" indicates agreement or acceptance rather than a relationship.

2. "It's raining much" is not a common way to express that it is raining. Instead, "much" is usually used as an adverb to modify a verb, indicating a large quantity. For example, you could say "It is raining much harder than before," which means it's raining to a greater degree or intensity than before.

3. "It's raining a lot" is a common expression used to convey that it is raining heavily or extensively. It implies a significant amount of rainfall.

4. "It's raining hard" is another way to express heavy rainfall. It emphasizes the intensity of the rain, suggesting that the rainfall is strong or forceful.

5. "It's raining heavily" is similar in meaning to "it's raining hard." Both expressions indicate a significant amount of rain falling at a considerable intensity.

6. "It's raining cats and dogs" is an idiomatic expression that means it's raining very heavily. It is an exaggerated expression indicating a torrential downpour.

Regarding using "much" instead of "a lot," these expressions have different grammatical structures. "A lot" functions as a noun phrase, while "much" is an adverb. So, you cannot directly replace "a lot" with "much" in the given sentences. However, you could rephrase the sentences to use "much" as an adverb. For example, "It is raining much" could be revised to "It is raining much more intensely" to convey a greater level of rainfall.