Nessuno vuole uscire quando piove così tanto.

Because nessuno is grammatically singular in Italian. It means no one / nobody, and it takes a singular verb.

• Nessuno vuole = No one wants
• Not Nessuno vogliono

This works like English no one wants, not no one want.

Nessuno by itself means no one / nobody when it comes before the verb:

• Nessuno vuole uscire. = No one wants to go out.

If nessuno comes after the verb, Italian usually also needs non:

• Non vuole uscire nessuno.

That has the same basic meaning, but the structure changes because nessuno is after the verb.

So in your sentence, Nessuno is at the start, so non is not needed.

Because after volere (to want), Italian normally uses an infinitive to express what someone wants to do.

• vuole uscire = wants to go out
• voglio mangiare = I want to eat
• vogliono dormire = they want to sleep

This is very similar to English want to + verb, except Italian does not use a separate word equivalent to to here.

Because volere is directly followed by the infinitive, without a preposition.

• vuole uscire
• voglio vedere
• vogliamo partire

Some Italian verbs do require a preposition before another verb, but volere does not.

So:

• Nessuno vuole uscire is correct
• Nessuno vuole a uscire is incorrect

In Italian, weather verbs are normally used without a subject pronoun.

• Piove = It’s raining
• Nevica = It’s snowing

Italian usually leaves out subject pronouns when they are not needed, and with weather expressions there is generally no pronoun like English it.

So English says it’s raining, but Italian simply says piove.

Yes. Quando here means when in the sense of whenever / in situations where.

So the sentence is talking about a general situation:

• quando piove così tanto = when it rains this much / when it’s raining so heavily

It is not necessarily referring to one specific moment. It can express a general truth or tendency.

Così tanto works together to mean so much / this much / so heavily.

• così = so / this
• tanto = much / a lot

Together, they intensify the amount:

• piove così tanto = it rains so much / it’s raining so heavily

You could also hear:

• piove tanto = it rains a lot
• piove molto = it rains a lot

But così tanto adds stronger emphasis, like so much in English.

Yes, in many situations you could say:

• Nessuno vuole uscire quando piove così molto — this is not natural
• Nessuno vuole uscire quando piove molto — natural
• Nessuno vuole uscire quando piove così tanto — natural and more emphatic

Important point: così combines naturally with tanto, but così molto is not the normal choice here.

So:

• piove molto = it rains a lot
• piove così tanto = it rains so much / this much

Yes, often you can, but uscire is the more direct and natural verb for to go out / to go outside.

• uscire = to go out
• andare fuori = to go outside / go out

In this sentence, uscire sounds very idiomatic and compact:

• Nessuno vuole uscire...

Using andare fuori is possible, but it may sound slightly less elegant depending on context.

The given word order is the most neutral and natural:

• Nessuno vuole uscire quando piove così tanto.

But Italian can move parts of the sentence around for emphasis. For example:

• Quando piove così tanto, nessuno vuole uscire.

This version puts more focus on the condition first: when it rains this much.

Both are correct. The difference is mainly one of emphasis and style.

It can easily be understood as a general statement: people in general do not want to go out when it is raining heavily.

Because of quando + present tense, Italian often expresses something habitual or generally true:

• Quando piove così tanto, nessuno vuole uscire.

Depending on context, it could also refer to the current situation, but by itself it sounds very general.

Because nessuno here is an indefinite negative pronoun meaning no one, not a noun phrase needing an article.

So Italian says:

• Nessuno vuole uscire.

Not:

• Il nessuno...
• Un nessuno... in this meaning

Be careful: un nessuno does exist in Italian, but it means something completely different, like a nobody / an insignificant person. That is not what is happening in your sentence.

In this sentence, it clearly modifies piove, not vuole.

Structure:

• Nessuno = subject
• vuole uscire = main idea
• quando piove così tanto = time/condition clause

Inside that clause, così tanto describes the amount or intensity of the rain:

• piove così tanto = it rains so much

So the meaning is not nobody wants so much to go out, but rather nobody wants to go out when it rains so much.