Magistra dicit vocalem in hoc vocabulo longam esse.

A word-by-word breakdown is:

• Magistra = the teacher (female teacher)
• dicit = says
• vocalem = a vowel / the vowel
• in hoc vocabulo = in this word
• longam = long
• esse = to be

So the structure is literally something like:

• The teacher says the vowel in this word to be long

Natural English: The teacher says that the vowel in this word is long.

Because after dicit Latin is using an indirect statement construction.

The noun vocalis means vowel. Its forms include:

• nominative singular: vocalis
• accusative singular: vocalem

In a normal direct statement, the subject would be nominative:

• Vocalis in hoc vocabulo longa est.
• The vowel in this word is long.

But after dicit, Latin changes the subject of the reported statement into the accusative, so vocalis becomes vocalem.

Because longam goes with vocalem.

The adjective longus, longa, longum must agree with the noun it describes in:

• gender
• number
• case

Here vocalem is:

• feminine
• singular
• accusative

So the adjective must also be feminine singular accusative:

• longam

That is why Latin has vocalem ... longam esse.

Because this is the standard Latin pattern for an indirect statement after a verb like dicit.

English says:

• The teacher says that the vowel is long.

Latin usually says:

• Magistra dicit vocalem longam esse.

So instead of a finite verb like est, Latin uses the infinitive:

• esse = to be

This is called the accusative-and-infinitive construction.

The indirect statement is:

• vocalem in hoc vocabulo longam esse

That whole part means:

• that the vowel in this word is long

Inside that indirect statement:

• vocalem is the subject of the reported idea
• longam is the predicate adjective
• esse is the infinitive verb

The main verb introducing it is:

• dicit = says

The direct statement would be:

• Vocalis in hoc vocabulo longa est.
• The vowel in this word is long.

Compare the two:

Direct:

• vocalis = nominative subject
• longa est = finite verb phrase

Indirect after dicit:

• vocalem = accusative subject
• longam esse = adjective + infinitive

So Latin changes the grammar when the statement is reported.

Hoc vocabulo is ablative singular.

That is because in with the ablative often means in or within a place or context:

• in hoc vocabulo = in this word

Here the forms are:

• hoc = ablative singular of hic, haec, hoc
• vocabulo = ablative singular of vocabulum

So the phrase tells us where the vowel is found: in this word.

Because the preposition in here takes the ablative, not the accusative.

The noun vocabulum is neuter, second declension:

• nominative/accusative singular: vocabulum
• ablative singular: vocabulo

Since Latin says in hoc vocabulo, both words are in the ablative singular:

• hoc matches vocabulo
• together they mean in this word

No. In this context it refers to vowel quantity, not physical length.

So vocalem longam means a long vowel: a vowel pronounced with greater duration.

This is the same distinction Latin grammarians care about in pronunciation and meter. So the sentence is about how the vowel is pronounced, not about size.

Because magistra is the subject of the main verb dicit.

The main clause is:

• Magistra dicit
• The teacher says

So magistra is nominative singular, since it is doing the action of saying.

The rest of the sentence is what she says.

Yes.

English normally says:

• The teacher says that the vowel in this word is long.

Latin usually does not use a separate word meaning that in this construction. Instead, it uses:

• accusative noun + infinitive

So vocalem ... longam esse is Latin’s way of expressing that the vowel ... is long.

Yes, Latin word order is flexible.

This sentence could be rearranged in different ways without changing the basic meaning, for example:

• Magistra dicit in hoc vocabulo vocalem longam esse.
• Vocalem in hoc vocabulo longam esse magistra dicit.

The original order is perfectly normal, but Latin often moves words around for emphasis or style.

Even so, the endings still show the grammar:

• magistra = nominative subject of dicit
• vocalem = accusative subject of the indirect statement
• longam = agreeing with vocalem
• hoc vocabulo = ablative after in

The nouns come from different declensions:

• magistra is first declension
• vocalis is third declension
• vocabulum is second declension neuter

That helps explain the forms:

• magistra = nominative singular
• vocalem = accusative singular of a third-declension noun
• vocabulo = ablative singular of a second-declension neuter noun

Because Latin has no definite or indefinite article.

So vocalem can mean:

• the vowel
• a vowel

In this sentence, English naturally uses the vowel, because the phrase in this word makes it specific.

Latin leaves that to context.

The big pattern is:

• verb of saying/thinking/perceiving + accusative + infinitive

Here that pattern is:

• dicit
  • vocalem
    • esse

With a predicate adjective added:

• vocalem longam esse

So a very useful rule is:

• After verbs like dicit, Latin often reports a statement by putting the subject in the accusative and the verb in the infinitive.

This sentence is a classic example of that pattern.