Cum ventus per speluncam flat, etiam pueri putant aliquid ibi sibilare.

Here cum means when or whenever.

So Cum ventus per speluncam flat means when/whenever the wind blows through the cave.

This is the very common temporal use of cum. Latin cum can also mean since, although, or while in other contexts, but here the sense is clearly temporal.

Because this is a straightforward temporal clause: when the wind blows...

With cum, Latin often uses:

• indicative for a simple time statement
• subjunctive for more circumstantial ideas, especially since, although, or background narrative when

So here:

• cum ... flat = simple when/whenever the wind blows
• not a more literary or causal/concessive cum clause

Flat is present indicative, 3rd person singular, from flare.

Ventus is nominative singular because it is the subject of flat.

• ventus = the wind
• flat = blows

So literally: the wind blows.

Because per means through, and per takes the accusative case.

So:

• per speluncam = through the cave

By contrast:

• in spelunca = in the cave (location, with ablative)
• in speluncam = into the cave (motion into, with accusative)

Here the idea is movement passing through the cave, so per speluncam is exactly what Latin wants.

Because it follows per, and per always governs the accusative.

The dictionary form is spelunca, -ae, feminine. In this sentence it appears as:

• speluncam = accusative singular

So the phrase is grammatically:

• per
  • accusative
• per speluncam = through the cave

Because etiam often stands near the word it emphasizes.

Here:

• etiam pueri = even the boys

That placement highlights pueri specifically. The sense is something like:

• even boys think this
• perhaps implying that not only very superstitious people, but even children, think so

If etiam were placed elsewhere, the emphasis could shift slightly, though the overall meaning would stay close.

Because after verbs of thinking, saying, knowing, hearing, and perceiving, Latin very often uses indirect statement, also called the accusative-and-infinitive construction.

So instead of:

• they think that something is hissing there

Latin says:

• putant aliquid ibi sibilare

Structure:

• putant = they think
• aliquid = subject of the indirect statement
• sibilare = infinitive, to hiss / to whistle

A very literal rendering would be:

• they think something to be hissing there

But in natural English we translate it with that:

• they think that something is hissing there

In this sentence, it functions as the accusative subject of the infinitive sibilare in indirect statement.

That said, aliquid is one of those neuter forms where nominative and accusative look the same. So the form itself does not change, but its job in the sentence is accusative.

You can tell from the construction:

• putant
  • accusative subject
    • infinitive

If the word were masculine instead, the accusative would look different more clearly.

Yes. Sibilare is the present active infinitive and means to hiss, to whistle, or to make a hissing sound.

It is infinitive here because of indirect statement after putant.

So:

• aliquid ibi sibilare = that something is hissing there

In English, we usually do not translate it with to hiss in this construction. We normally say:

• they think that something is hissing there

But the Latin grammar is built with the infinitive.

Ibi means there.

It modifies sibilare in sense:

• aliquid ibi sibilare = something is hissing there

Latin adverbs like ibi are fairly flexible in position. Here it sits naturally between aliquid and sibilare, but Latin could move it around without changing the basic meaning very much.

Its position here is ordinary and unobtrusive.

Latin word order is much freer than English word order because Latin endings show grammatical function.

This sentence is quite natural as written:

• Cum ventus per speluncam flat, setting the scene first
• etiam pueri, with emphasis on even the boys
• putant aliquid ibi sibilare, the main thought

A Latin author could rearrange parts for emphasis, for example:

• Etiam pueri, cum ventus per speluncam flat, aliquid ibi sibilare putant.

That would still mean essentially the same thing.

So yes, the order is flexible, but the chosen order is idiomatic and clear.

The main finite verbs are:

• flat — present indicative
• putant — present indicative

And the infinitive is:

• sibilare — present infinitive

The present tense here gives a general or habitual sense:

• when/whenever the wind blows through the cave, even the boys think something is hissing there

So this is not necessarily one single event at one exact moment. It can describe a repeated or typical situation.

It can be understood as either, depending on context, but whenever is often a very helpful way to feel the sense here.

Because the sentence sounds like a general truth or repeated experience, many learners would naturally understand:

• Whenever the wind blows through the cave, even the boys think something is hissing there.

If the broader context were about one specific occasion, then simple when would fit better. Both are compatible with the Latin here.