Pater rogat quanta summa pecuniae supersit.

The sentence has two parts:

• Pater rogat = The father asks
• quanta summa pecuniae supersit = an indirect question, roughly how great an amount of money remains

So the structure is:

• main clause: Pater rogat
• subordinate clause: quanta summa pecuniae supersit

Latin often uses an indirect question after verbs like ask, know, see, find out, and so on.

Quanta means how great or how large, and here it is being used to ask about amount.

It agrees with summa, which is feminine singular, so it must also be feminine singular:

• quantus = masculine
• quanta = feminine
• quantum = neuter

Because summa is feminine, Latin says quanta summa = how large an amount.

Summa means sum, total, or amount.
Pecuniae is a genitive singular, so summa pecuniae literally means the sum of money or the amount of money.

This is a very common Latin pattern:

• summa pecuniae = amount of money
• multitudo hominum = multitude of people
• pars urbis = part of the city

So pecuniae depends on summa and tells you an amount of what.

Because it is not the main noun of the clause. The main noun is summa.

Latin uses the genitive after many nouns to show a relationship like of in English. So:

• summa = amount
• pecuniae = of money

That gives amount of money.

If pecunia were nominative, it would try to function as a subject. If it were accusative, it would look like a direct object. But here Latin wants of money, so it uses the genitive: pecuniae.

The subject is summa.

So literally the indirect question is:

• quanta summa pecuniae supersit = how great a sum of money remains

Even though pecuniae is nearby, it is not the subject; it is genitive and depends on summa.

Because this is an indirect question, and Latin normally puts the verb of an indirect question into the subjunctive.

Compare:

• direct question: Quanta summa pecuniae superest?
  = How much money remains?

• indirect question: Pater rogat quanta summa pecuniae supersit.
  = Father asks how much money remains.

So superest is the indicative form you would expect in a direct statement or direct question, while supersit is the subjunctive used in the indirect question.

Supersit is present subjunctive.

It comes from supersum, superesse, superfui = to remain, to be left over.

The present subjunctive is used here because the main verb rogat is present, and the remaining is happening at the same time as the asking. In other words:

• He asks now
• how much money remains now

So the tense relationship is same time as the main verb, and because the main verb is in a primary tense (rogat), Latin uses the present subjunctive.

It comes from supersum, which is a compound of super- + sum.

Because it is built on sum, many of its forms resemble forms of sum:

• superest = he/she/it remains
• supersunt = they remain
• supersit = he/she/it may remain / remains in an indirect question

So supersit may look unusual, but that is because supersum follows the pattern of sum.

Yes. More specifically, it is an indirect question functioning as the object of rogat.

The verb rogat needs something to complete its meaning: he asks what? The answer is the whole clause:

• quanta summa pecuniae supersit

So the entire indirect question acts as the content of what he asks.

The direct question would be:

Quanta summa pecuniae superest?

That is the question someone would ask directly: How much money remains?

When Latin reports that question indirectly after rogat, it changes:

• superest → supersit

So:

• direct: Quanta summa pecuniae superest?
• indirect: Pater rogat quanta summa pecuniae supersit.

Yes, Latin word order is flexible. This order is natural and clear, but other arrangements are possible.

The given order emphasizes the sentence nicely:

• Pater rogat first gives the main action
• then quanta summa pecuniae introduces the indirect question
• supersit comes at the end, where Latin often places the verb in a subordinate clause

You could rearrange the words and still keep the same basic meaning, as long as the endings stay the same. But the original order is good standard Latin.

No, not here.

Quantum would be neuter singular, but summa is feminine singular. Since quanta modifies summa, it must agree with it in gender, number, and case.

So Latin needs:

• quanta summa = feminine singular + feminine singular

not:

• quantum summa

Agreement is one of the key things to watch in Latin.

That is just a difference in expression between Latin and English.

English often uses how much money. Latin can express the same idea through a noun phrase like:

• quanta summa pecuniae = how large a sum of money

This is a very idiomatic Latin way to talk about quantity or amount. It may sound a bit more formal or concrete than everyday English, but it is perfectly normal Latin.