Cursus pueri canem terret.

Because cursus is in the nominative case, which is the case normally used for the subject of the sentence.

Here the forms point us in that direction:

• cursus = nominative singular
• canem = accusative singular, so it is the direct object
• terret = he/she/it frightens, a singular verb

So the thing doing the frightening must be a singular subject, and cursus fits that role.

By contrast, pueri here is best taken as genitive singular: of the boy.

Because pueri is the genitive singular form of puer (boy).

The genitive often shows possession or close relationship, so:

• pueri = of the boy / the boy’s

So cursus pueri means something like:

• the boy’s running
• the running of the boy

In smoother English, you would usually say the boy’s running.

Yes, pueri can also be nominative plural and mean boys. That is a very common source of confusion.

But in this sentence, that reading does not work well grammatically:

• terret is singular, not plural
• if pueri were the subject meaning boys, you would expect a plural verb such as terrent

So here pueri is understood as genitive singular, not nominative plural.

Because canem is in the accusative singular, the case commonly used for the direct object.

The dictionary form is canis (dog), but in the accusative singular it becomes:

• canem = dog as the thing affected by the action

So in this sentence, the dog is the one being frightened.

Terret comes from terreō, terrēre, meaning to frighten.

The ending -t tells you it is:

• 3rd person
• singular
• present tense

So terret means:

• he frightens
• she frightens
• it frightens

Since cursus is not a person, English will normally translate it as it frightens.

Latin word order is much freer than English word order because Latin uses case endings to show each word’s function.

English depends heavily on position:

• The boy frightens the dog is different from
• The dog frightens the boy

Latin can move words around more easily because the endings already show who is doing what.

So Cursus pueri canem terret can still be understood as:

• subject: cursus
• related genitive: pueri
• object: canem
• verb: terret

The order may also help emphasize certain words or create a natural Latin rhythm.

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

So a noun like canem can mean:

• a dog
• the dog

and pueri can mean:

• of a boy
• of the boy

You decide from context which English article sounds right. That is why translation into English often adds the or a even though no separate Latin word is present.

Cursus is a 4th-declension noun.

Its dictionary form is usually given as:

• cursus, -ūs (masculine)

It can mean things like:

• running
• course
• movement
• race

In this sentence, it is being used as a singular noun, and the most natural sense is something like the boy’s running.

Because Latin often uses a noun where English prefers a more natural expression.

Word-for-word, cursus pueri could be the course/running of the boy, but idiomatic English would usually say:

• the boy’s running

So even if the literal structure is noun + genitive, the best English translation may be smoother and less literal.

The verb terret is singular, and the main subject cursus is singular too.

The sentence is built around one action done by one subject:

• cursus = one running/course
• terret = frightens, singular
• canem = one dog

Even though pueri looks like it could be plural, the rest of the sentence shows that here it is not functioning as a plural subject.