Prope speluncam cautius ambulatur, quia serpentes ibi interdum sibilare possunt.

Because prope is a preposition that takes the accusative.

So:

• prope = near
• speluncam = cave, in the accusative singular

That is why Latin says prope speluncam for near the cave.

A learner might expect something like the ablative after a preposition of place, but prope is one of the prepositions that regularly governs the accusative.

Ambulatur is an impersonal passive form.

It is literally something like:

• it is walked
• or more naturally in English: people walk, one walks, walking happens

Latin often uses the passive of an intransitive verb impersonally in the 3rd person singular to express a general action with no named subject.

So:

• ambulant = they walk
• ambulatur = people walk / one walks / there is walking

In this sentence, the idea is general: near the cave, one walks more carefully.

In Latin, some intransitive verbs can form an impersonal passive.

Since ambulo does not normally take a direct object, it cannot have an ordinary personal passive like the road is walked in the way a transitive verb can have the book is read. But Latin still allows a passive form in an impersonal sense:

• ambulatur = people walk / there is walking

This is a normal Latin construction, even though it sounds strange if translated too literally into English.

Cautius means more cautiously or more carefully.

It is the comparative adverb of caute.

The pattern is:

• cautus = careful, cautious
• caute = carefully, cautiously
• cautius = more carefully, more cautiously

So cautius ambulatur means people walk more cautiously.

The comparative does not always have to mean an explicit comparison with something stated in the sentence. It can simply suggest with extra caution.

Because it is modifying the verb ambulatur, not a noun.

Ask what word cautius describes:

• Does it describe a person or thing? No.
• Does it describe the action of walking? Yes.

So it must be an adverb:

• careful person = adjective
• walk carefully = adverb

Here, cautius tells us how the walking is done.

There is no expressed subject in the usual sense.

That is the point of the impersonal construction. Latin uses ambulatur to mean something like:

• one walks
• people walk
• it is customary to walk

So the sentence does not say who exactly is walking. It gives a general statement about what people do near the cave.

Because serpentes is the subject of possunt.

Break that part down:

• serpentes = snakes
• possunt = are able / can
• sibilare = to hiss

So:

• serpentes ... possunt = snakes can ...
• sibilare = hiss

Therefore serpentes must be nominative plural, since it is the subject performing the action.

Because possum is a modal verb meaning to be able or can, and it is followed by an infinitive.

So:

• sibilant = they hiss
• sibilare possunt = they can hiss

This is very similar to English:

• snakes hiss
• snakes can hiss

The infinitive sibilare depends on possunt.

Quia introduces a clause of reason: because.

So the structure is:

• main clause: Prope speluncam cautius ambulatur
• reason clause: quia serpentes ibi interdum sibilare possunt

That means:

• people walk more cautiously near the cave,
• because snakes there can sometimes hiss

Quia is one of the standard Latin words for because.

Ibi means there.

It points back to the place already mentioned, namely near the cave. So the sense is:

• snakes can sometimes hiss there
• that is, in that place / near the cave

Latin often uses a word like ibi even when English might leave it out or express it less directly.

Interdum means sometimes or from time to time.

It tells us that the snakes do not hiss constantly, but that it happens on occasion.

So:

• ibi interdum sibilare possunt = they can sometimes hiss there

It is an adverb of frequency.

The word order is perfectly natural for Latin, even though it is freer than English word order.

The sentence is:

• Prope speluncam — sets the scene first: near the cave
• cautius — highlights the manner: more cautiously
• ambulatur — gives the main verb
• quia — introduces the reason
• serpentes — names the subject of the reason clause
• ibi interdum — adds there and sometimes
• sibilare possunt — finishes with the verbal idea can hiss

Latin often places words for emphasis or flow rather than following a rigid English-style order. The meaning comes mainly from the forms of the words, not just their position.

Its basic meaning is near the cave or close to the cave.

Whether that means just outside it or simply in its vicinity depends on context. By itself, prope does not necessarily mean immediately at the entrance; it simply indicates nearness.

So the phrase is best understood broadly as in the area near the cave.

Because they are doing two different jobs.

• ambulatur is an impersonal passive, which is normally 3rd person singular
• possunt has a real subject, serpentes, so it is plural

So:

• ambulatur = one walks / people walk
• serpentes possunt = snakes can

The singular in ambulatur does not mean only one person walks. It is singular because the construction is impersonal.

A very literal translation would be:

• it is walked more cautiously

But that is not natural English. Better English translations are:

• one walks more cautiously
• people walk more cautiously
• walking is done more cautiously

This is a good example of where understanding the grammar matters more than translating word for word.