Cum tonitrus auditur, canis sub lecto iacet.

Here cum means when. It is introducing a time clause: Cum tonitrus auditur = when thunder is heard.

This is not the preposition cum meaning with. You can tell because it is followed by a whole clause, not just a noun.

• cum amico = with a friend
• cum tonitrus auditur = when thunder is heard

Because this is a straightforward temporal cum clause: it simply tells you when something happens.

So Latin uses the indicative:

• auditur = present indicative passive

A cum clause can use the subjunctive in other situations, especially in more literary Latin, but here the idea is just simple time: when thunder is heard.

Because tonitrus is the subject of the passive verb auditur.

In English, we often think of thunder as the thing someone hears, so we might expect an object. But in Latin this clause is passive:

• auditur = is heard

So the thing being heard becomes the grammatical subject:

• tonitrus auditur = thunder is heard

That is why tonitrus is nominative, not accusative.

Tonitrus is nominative singular.

It is the subject of auditur. A learner might assume it is a 2nd-declension noun because it ends in -us, but this word is actually commonly treated as a 4th-declension masculine noun.

So here the important point is:

• tonitrus = nominative singular
• it is the subject of auditur

Auditur is the 3rd person singular present passive of audire.

So it means:

• is heard
• or more naturally in English, sometimes one hears

Literally:

• tonitrus auditur = thunder is heard

Latin often uses the passive in places where English might prefer a more natural active expression.

Canis is nominative singular, and it is the subject of iacet.

So in the main clause:

• canis = the dog
• iacet = lies

Together:

• canis ... iacet = the dog lies ...

Also, canis is a 3rd-declension noun.

Yes. Canis can refer to either, depending on context.

Latin often does not force you to specify that unless the context makes it important. So canis may simply mean dog.

Because sub can take two different cases depending on the meaning.

• ablative = location, where something is
• accusative = motion, where something is going

Here the dog is already under the bed, not moving there:

• sub lecto = under the bed

If there were movement toward that position, Latin could use the accusative instead.

Lecto is ablative singular of lectus, meaning bed.

It is ablative because it follows sub in the sense of location:

• sub lecto = under the bed

Iacet means lies or is lying.

It is the 3rd person singular present active of iacere in the sense to lie down / to be lying.

So:

• canis sub lecto iacet = the dog lies under the bed

No. They come from different verbs.

• iacet from iaceo = lies
• iacit from iacio = throws

That difference is very important. In this sentence, iacet definitely means lies, not throws.

The present tense here expresses a general or repeated situation:

• When thunder is heard, the dog lies under the bed

So the idea is not necessarily one single event at one single moment. It can mean something habitual or typical.

In English, we often translate this kind of present as:

• when
• or whenever

depending on context.

Because Latin has no articles.

So a noun like canis can mean:

• a dog
• the dog

and lecto can mean:

• a bed
• the bed

The context tells you which English article makes best sense.

Very literally, it is:

• when thunder is heard

But English often prefers a less literal version such as:

• when there is thunder
• when one hears thunder
• when it thunders

The Latin grammar is still passive, though, so the literal structure is useful to understand.

Latin word order is much more flexible than English word order because the endings show how words function.

This sentence puts the time clause first:

• Cum tonitrus auditur

Then the main clause:

• canis sub lecto iacet

Also, Latin often likes to place the verb near the end of the clause, as happens here with iacet.

So the word order is normal and natural for Latin, even though it does not match English word-for-word.