Canis retro ianuam latet, quia tonitrum audit.

Because retro here is being used like a preposition meaning behind, and with that use it takes the accusative.

So:

• retro ianuam = behind the door

This may feel unusual to an English speaker, because English does not change the form of door after behind, but Latin often shows these relationships through case endings.

Retro can mean backward, back, or behind, depending on how it is used.

In this sentence, it means behind:

• retro ianuam = behind the door

A learner may first meet retro as an adverb, but Latin can also use it with an object, almost like a preposition.

Yes, post ianuam could also mean behind the door in many contexts.

However, retro ianuam is perfectly good Latin and has a slightly more literal sense of at the back of / behind. A learner should mainly notice that both can express position behind something, though authors may prefer one over the other depending on style and context.

Canis is the nominative singular form, so it is the subject of the sentence:

• canis = the dog / a dog

This tells us that the dog is the one doing the actions:

• latet = hides
• audit = hears

It can be either masculine or feminine, depending on the actual dog.

That is common with some Latin nouns referring to animals or people. In this sentence, nothing forces us to decide whether the dog is male or female, so English simply says the dog.

Latet comes from lateo, latere, meaning to lie hidden, to be hidden, or to hide in the sense of being concealed.

So canis ... latet means:

• the dog is hiding
• or more literally, the dog lies hidden

This is a little different from an active verb like celat, which means hides something.

Latet is the ordinary present form of lateo: he/she/it hides or is hidden.

Latitat exists too, from latito, and often suggests keeps hiding, lurks, or hides repeatedly/frequently. That would give a slightly different nuance. In your sentence, latet is the simpler and more neutral choice.

Because the sentence describes what is happening now, or what generally happens:

• canis ... latet = the dog hides / is hiding
• quia tonitrum audit = because it hears thunder

Latin present tense often covers both simple present and present progressive in English, so audit can mean hears or is hearing, depending on context.

Tonitrum is accusative singular here, because it is the direct object of audit:

• audit tonitrum = hears thunder

A useful thing to notice is that tonitrum is a neuter noun, and many neuter nouns have the same form in the nominative and accusative singular. So the form looks the same, but here its job in the sentence is accusative.

It can mean thunder, thunderclap, or the sound of thunder.

In this sentence, the broad idea is simply that the dog hears thunder. English can translate it naturally as:

• because it hears thunder
• or because it hears a thunderclap

Quia means because, introducing a reason:

• quia tonitrum audit = because it hears thunder

It is one of the common ways to express cause in Latin. A learner may later meet other causal words such as quod or quoniam, but quia is straightforward and very common.

Because this sentence gives a straightforward, real reason:

• because the dog hears thunder

So Latin uses the indicative:

• audit

In many ordinary causal clauses, especially with quia, the indicative is exactly what you would expect. The subjunctive would usually suggest a different nuance, often involving reported thought, alleged reason, or a more subjective perspective.

Latin word order is more flexible than English because case endings show the grammatical relationships.

So all of these would still be understandable Latin:

• Canis retro ianuam latet
• Retro ianuam canis latet
• Latet canis retro ianuam

The chosen order is natural and clear. It presents:

1. the subject — canis
2. the location — retro ianuam
3. the verb — latet

Then the reason follows: quia tonitrum audit.

Latin has no articles like English the and a/an.

So canis can mean:

• a dog
• the dog

And ianuam can mean:

• a door
• the door

The context tells you which is most natural. Since you already know the sentence’s meaning, the translator has chosen the most suitable English wording.

It could potentially mean either, because Latin present tense is flexible. But in this sentence the most natural interpretation is immediate and concrete:

• the dog is hiding because it hears thunder now

So the whole sentence paints a little scene: the dog is behind the door, hiding, because thunder is frightening it.

Here it describes location: where the dog is hiding.

• retro ianuam latet = it is hiding behind the door

Even though English speakers often expect location words to work one way and motion words another, with retro the accusative can still appear in this positional sense. The important thing is to read the phrase as a unit meaning behind the door.