Puer rogat unde sonus veniat; avus respondet aquam de monte ad fontem currere.

Because unde sonus veniat is an indirect question after rogat.

In Latin, indirect questions normally use the subjunctive, so:

• veniat = present subjunctive
• not venit = indicative

Compare:

• Unde sonus venit? = a direct question: Where does the sound come from?
• Puer rogat unde sonus veniat. = an indirect question: The boy asks where the sound comes from.

So the subjunctive here is not making the idea unreal; it is simply the normal grammar for an indirect question.

Unde means from where or where ... from.

That is different from:

• ubi = where (location)
• quo = to where / where ... to
• unde = from where / where ... from

So:

• unde sonus veniat = where the sound is coming from

This fits well with venire, since English also often says come from.

Yes. Sonus is the subject of veniat.

You can see this because:

• sonus is nominative singular
• veniat is 3rd person singular

So the structure is:

• unde = from where
• sonus = the sound
• veniat = may come / comes, in an indirect question

In other words, the sound is the thing that is coming.

Because after a verb like respondet, Latin often uses an indirect statement rather than a clause with that.

English says:

• The grandfather answers *that water runs ...*

Latin often says this with:

• accusative + infinitive

So:

• aquam ... currere = that water runs / is running ...

If Latin used a finite verb here, it would be a different construction. The infinitive is the normal way to express reported statement after many verbs of saying, thinking, knowing, perceiving, and here also respondet.

It is the subject of currere, but in an indirect statement, the subject of the infinitive goes into the accusative.

So in:

• aquam de monte ad fontem currere

aquam is the subject of the infinitive currere, even though it is accusative.

This is one of the big differences between English and Latin:

• English: he says *that water runs*
• Latin: he says *water-ACC to run*

This pattern is often called the accusative-and-infinitive construction.

• monte is ablative singular
• fontem is accusative singular

They are in those cases because of the prepositions:

• de monte = from the mountain
  • de takes the ablative
• ad fontem = to the spring/fountain
  • ad takes the accusative

So the phrase shows movement:

• de monte = starting point
• ad fontem = destination

Because Latin usually does not need a separate word like English that in this kind of sentence.

English often says:

• The grandfather answers *that water runs ...*

Latin normally expresses this idea by the accusative + infinitive construction:

• respondet aquam ... currere

So the meaning of English that is built into the grammar of the indirect statement, rather than shown by a separate word.

Yes, Latin word order is much more flexible than English word order because the endings show each word’s job.

For example:

• sonus is nominative, so it is the subject
• aquam is accusative, so it belongs to the indirect statement as the infinitive’s subject
• monte is ablative after de
• fontem is accusative after ad

Because of this, Latin can move words around for emphasis or style.

In this sentence:

• aquam comes early in the second clause, which gives it some prominence
• currere comes at the end, which is very natural in Latin

So the order is normal and readable, but not the only possible order.

Because Classical Latin has no articles.

So:

• puer can mean the boy or a boy
• avus can mean the grandfather or a grandfather
• sonus can mean the sound or a sound
• fontem can mean the spring/fountain or a spring/fountain

English must choose between the and a/an, but Latin usually leaves that to context.

Yes, and that is a useful thing to notice.

Both de monte and ex monte can mean from the mountain, but there can be a slight nuance:

• de often suggests down from or off/from
• ex often suggests out of/from

So de monte can feel very natural for movement down from a mountain. In many contexts, though, the difference is small, and both can be translated simply as from the mountain.