Casus puellae matrem terret.

The subject is casus.

You can tell because:

• terret means frightens or is frightening, and it is 3rd person singular
• the noun that goes with a singular verb as the doer of the action is casus
• casus here is nominative singular, the case normally used for the subject

So the basic structure is:

• casus = the thing doing the frightening
• matrem = the person being frightened

Because puellae is most naturally genitive singular here, and the genitive often means of ....

So:

• puellae = of the girl
• casus puellae = the girl's misfortune/accident or the misfortune of the girl

A learner often notices that puellae can mean several things in different contexts, and that is true. It can be:

• genitive singular = of the girl
• dative singular = to/for the girl
• nominative plural = girls

But in this sentence, of the girl makes the best grammatical and logical sense.

Because matrem is accusative singular, and the accusative is the normal case for the direct object.

The verb terreō, terrēre means to frighten, and the person or thing frightened goes into the accusative.

So:

• matrem = mother in the accusative
• therefore matrem is the mother as the direct object

In other words, the sentence says that something frightens the mother.

Because Latin does not decide the subject mainly by word order. It decides it mainly by case ending.

Even though puellae appears early, its form does not fit the role of subject here as well as casus does.

A native English speaker may expect the first noun to be the subject, but Latin is much freer with word order. In this sentence:

• casus is nominative singular and matches terret
• puellae is best taken as genitive singular, depending on casus
• matrem is accusative singular

So puellae is part of the phrase casus puellae, not the subject by itself.

Because Latin relies much more on endings than on word order.

English usually needs a fixed order:

• The girl's misfortune frightens the mother

Latin can arrange the words more freely because the endings already show who is doing what to whom.

So Casus puellae matrem terret still means:

• casus = subject
• puellae = dependent genitive
• matrem = object
• terret = verb

Also, Latin often places the verb at the end, which is exactly what happens here.

They come from three different declensions:

• casus — 4th declension
• puella — 1st declension
• mater — 3rd declension

Their forms in this sentence are:

• casus = nominative singular
• puellae = genitive singular
• matrem = accusative singular

This is a good example of why Latin learners must pay attention to declension patterns, not just word order.

Terret is a verb form from terreō, terrēre, meaning to frighten.

Specifically, terret is:

• present tense
• active voice
• indicative mood
• 3rd person singular

So it means:

• he frightens
• she frightens
• it frightens

In this sentence, since casus is not a person, the natural English translation is it frightens or simply frightens.

In isolation, yes. But in this sentence, probably not.

As a form, puellae can be:

• genitive singular = of the girl
• dative singular = to/for the girl
• nominative plural = girls

Why not those other meanings here?

• If it were nominative plural (girls), then we would expect a plural verb, but the verb is terret, singular.
• If it were dative singular (to/for the girl), the sentence would be much harder to make sense of with terret, and it would leave casus without a clear dependent relationship.

So genitive singular is the best reading.

No. Latin has no definite article like English the, and no indefinite article like a/an either.

So casus puellae matrem terret could be translated in different article-choices depending on context, for example:

• The girl's misfortune frightens the mother
• A girl's misfortune frightens the mother
• The misfortune of the girl frightens the mother

English must choose an article, but Latin usually does not state one.

Yes. Casus puellae forms a noun phrase.

• casus = the main noun
• puellae = a genitive modifying it

So puellae tells you whose casus it is.

This is very common in Latin: a genitive noun attached to another noun to show possession or close relationship.

Examples in English style:

• the girl's book
• the mother's fear
• the city's walls

Here it is:

• the girl's misfortune/accident

Yes, as long as the endings stay the same.

For example, these would mean essentially the same thing:

• Casus puellae matrem terret
• Matrem casus puellae terret
• Puellae casus matrem terret

The emphasis may shift slightly, but the core meaning stays the same because:

• casus is still nominative
• puellae is still genitive
• matrem is still accusative
• terret is still singular

That is one of the big differences between Latin and English.