Puer rogat quomodo mustum ex uvis in torculari fiat.

Because Latin often uses fio, fieri as the passive equivalent of facio, facere.

So:

• facit = he/she/it makes
• fit = he/she/it is made / happens
• fiat = may be made / is made in a subjunctive context

In this sentence, mustum ... fiat means how grape juice/must is made.

So although English uses is made, Latin does not use a regular passive form of facere here; it uses fio instead.

Because quomodo mustum ex uvis in torculari fiat is an indirect question: the boy asks how ... is made.

In Latin, indirect questions normally take the subjunctive, not the indicative. That is why you get:

• direct question: Quomodo mustum fit? = How is must made?
• indirect question: Puer rogat quomodo mustum fiat. = The boy asks how must is made.

So fiat is present subjunctive, used because of the indirect question introduced by quomodo.

Quomodo means how or in what way.

It introduces the indirect question:

• quomodo ... fiat = how ... is made

So the structure is:

• Puer rogat = The boy asks
• quomodo mustum ex uvis in torculari fiat = how must is made from grapes in the press

It is the Latin equivalent of English how in a clause like He asks how it is done.

Mustum means must, that is, freshly pressed grape juice before it becomes wine. In beginner materials it is often understood as grape juice or new wine depending on context.

Grammatically, mustum is neuter singular and is the subject of fiat:

• mustum fiat = the must is made / must comes into being

Even though mustum comes before the prepositional phrases, it is still the thing being produced.

Ex means out of or from, and it takes the ablative case.

So:

• uva = grape
• uvis = from grapes / out of grapes (ablative plural)

That is why Latin says:

• ex uvis = from grapes

This tells you the material or source from which the must is produced.

Torculari is ablative singular after the preposition in.

Here in means in in the sense of location, not motion toward. When in means location, it takes the ablative:

• in torculari = in the press

A torcular is a press, especially an olive press or wine press. In this sentence it refers to the place where the grapes are pressed.

So:

• in torculari = in the wine press

Puer is the subject of the main verb rogat:

• Puer rogat = The boy asks

But inside the indirect question, the subject is mustum:

• mustum ... fiat = the must is made

So the sentence has two layers:

1. main clause: Puer rogat
2. subordinate indirect question: quomodo mustum ex uvis in torculari fiat

This is very common in Latin.

Because this is not a that-clause but an indirect question.

Compare:

• Puer dicit mustum bonum esse. = The boy says that the must is good.
• Puer rogat quomodo mustum fiat. = The boy asks how the must is made.

The second one is built with a question word, quomodo, so Latin simply introduces the clause with that word and uses the subjunctive.

English works similarly:

• He asks how it is made, not He asks that how it is made

The word order is fairly natural for Latin, but Latin is more flexible than English.

The sentence is:

• Puer = subject first
• rogat = main verb
• quomodo = introduces the indirect question
• mustum = the thing being made
• ex uvis = source/material
• in torculari = place
• fiat = verb of the subordinate clause, placed at the end

Putting the verb at the end of the clause is very common in Latin. English would usually say:

• The boy asks how must is made from grapes in the press.

Latin often saves the verb for later, especially in subordinate clauses.

Yes, fio can mean be made, become, or happen, depending on context.

For example:

• quid fit? = what is happening?
• panis fit ex farina = bread is made from flour

In this sentence, the context makes is made clearly the right meaning:

• mustum
• ex uvis
• in torculari

These words describe a production process, so fiat means is made rather than simply happens.

Yes. A more literal translation would be:

• The boy asks in what way must comes to be from grapes in the press.

But that sounds unnatural in English. A smoother translation is:

• The boy asks how must is made from grapes in the press.

Literal translations can help you see the grammar, but natural translations are usually better English.