Note that "What for?" is informal/conversational.
"What Purposeful Reason?"
"What for" denotes a purposeful reason, while "Why?" can be used for causes, reasons, or explanations.
- Statement 1: "I am going to work now."
- Why? (Explanation): "Because it's time for me to leave."
Why? OR What for? (Purpose): "To make money."
Statement 2: "Things fall."
- Why (explanation): "Because of gravity..."
- Why (Purpose): "Because of gravity." / "No, I mean, why is it like that? Why is there gravity?" / "Who knows? That's just the way it is. God? Quantum Multiverse? That may not be answerable."
What for (purpose only!): "Who knows? God? Quantum Multiverse? That may not be answerable."
Statement 3: "I exist."
- Why (Explanation/Cause): "Because your parents had sex, your mother got pregnant, and you were born."
- Why OR What for? (Purpose): "In order to propagate the species." OR "For God's mysterious reason." OR "Nobody knows." OR "There is no purpose; it's a quantum multiverse."
Variations can be created by sentences like "What is $NounPhrase$ for?" For example, if one is in a new car with a salesman, one can ask "What is that button for?" In this case, the question is asking for the functional purpose of the button. Also, "What is math good for?" is asking for suitability for a purpose.
"What for!?" -- Emphasis / Surprise / Suspicion
"What for" can lend itself to more emotion than a simple "Why", adding surprise, suspicion, or just more emphasis. In this case, "what for" can be asking for an explanation just like "why":
- Statement 4: "I am going to work now."
- What for!? You just got home! (Surprise/Explanation): / "Because I left my wallet there."
The emotion behind "What for?" can also be suspicion or interrogation. There can be a bit of eye squinting or head tilting:
"Can I ask you your name?"
"Ummm. What for?" (Suspicion)
"For what?" -- Identifying ambiguity.
"What for" can also be used to mean "For what?" in order to identify an ambiguous reference. The emphasis is on replacing the "what" with the named item rather than replacing the "for" with the named reason:
- "I'm looking for something." / "For what?" / "For my glasses."
- "I'm looking for something." / "What for?" / "My glasses." (Same as "For what")
- "What are you looking for?" / "My glasses."
Here's another one:
- "I need some help." / "What for?" / "To reach that top shelf." / "Ok, what for?" / "That coffee mug."
Both "what for" questions could be asking for purposeful reason. For example, in the 2nd case, the reason could have been to clean the shelf. But the answer given in the 2nd case was an identification of the object desired.
Also, "What am I responsible for?" is asking for identification of responsibilities. It is like "For what am I responsible?", but the fronted what sounds more natural.
Use "much" to describe a relative amount of a noun that can't be easily counted or quantified. If you can't put a number before the noun, use "much."
Also, if you replace the object with a pronoun, it should be singular (it). Consider "collective nouns" that are singular but describe an unquantifiable volume.
"Much" is often combined with a modifier like "too," "not," or "very."
There is not much snow on the ski slopes this year. It's a small amount.
There's much work ahead before the end of the project. It's a big one!
There's too much sand on the beach to count it all. It's everywhere!
"Many" describes a quantifiable, countable noun. If you can put a number before the noun you should probably use "many." Here if you replace the object with a pronoun, it will be plural (them, they).
Many ski-lift operators will be looking for jobs. Who will employ 30 of them?
This project has many complex parts to consider. They are all critical - all 1000 parts.
There are too many grains of sand on the beach to count them all. At least a million of them are in my left shoe.
Using "a lot of" or "lots of" is sort of personal preference. In most cases the 2 are interchangeable.
There is a lot of ice on the road, so drive carefully! Also, there's lots of snow!
We still have a lot of work to do - lots of separate tasks to complete.
I found a lot of sand in my right shoe, and lots more in places I didn't know existed.
A lot of people make the mistake of writing "alot," so don't do that!
In most cases either one works fine, but you should be careful about replacing "a lot" or "lots" with "many" or "much." Note that in the examples below "many" works in place of "lots/a lot", but "much" doesn't work at all.
At first there was just one monkey, but then the banana truck exploded and there were lots of monkeys running all over the place!
A lot of the monkeys ran off to the beach afterward. Lots of bananas still litter the road though.
The town will need to hire a lot of people to clean them up. Lots of people need jobs now anyway.
Best Answer
Compulsory (“Required; obligatory; mandatory”), mandatory (“Obligatory; required or commanded by authority”), and obligatory (“Imposing obligation, morally or legally; binding”) have related and similar meanings.
There are many examples where one of those words could be used in place of another, but in some areas, set phrases arise; for example, compulsory education, compulsory service, mandatory minimum sentences, mandatory retirement, obligatory treatment. For further examples, click the book links at ngrams for compulsory,mandatory,obligatory. Also see ELU question Are the words “mandatory,” “obligatory,” and “compulsory” interchangeable? and similar questions at other sites (1, 2, 3, 4, 5). The differencebetween page is quite informative: