Magistra dicit nominativum saepe in initio sententiae poni.

Because this sentence uses indirect statement after dicit.

After verbs like dicit (says), Latin often uses:

• an accusative for the subject of the reported statement, and
• an infinitive for the verb.

So the underlying idea is:

• direct idea: Nominativus saepe in initio sententiae ponitur.
  = The nominative is often placed at the beginning of the sentence.

But after dicit, that becomes:

• Magistra dicit nominativum ... poni.
  = The teacher says that the nominative ... is placed

A small detail: nominativum is neuter singular, and in neuter nouns/adjectives the nominative and accusative forms are often identical in shape. So a learner may not immediately see the case change, but syntactically it is functioning as the accusative subject of the infinitive.

For the same reason: this is an indirect statement.

In a normal finite clause, you would say:

• nominativus ponitur = the nominative is placed

But after dicit, Latin changes the reported verb into an infinitive:

• poni = to be placed

So:

• Magistra dicit nominativum poni
  literally = The teacher says the nominative to be placed
  natural English = The teacher says that the nominative is placed

Also, poni is the present passive infinitive of pono.

It is the accusative-and-infinitive construction, often called indirect statement.

Pattern:

• verb of saying / thinking / knowing
• accusative subject
• infinitive verb

Here:

• dicit = verb of saying
• nominativum = accusative subject of the reported statement
• poni = infinitive

So the reported statement is:

• nominativum saepe in initio sententiae poni

This is one of the most important Latin constructions, and English speakers often need time to get used to it because English usually uses that + finite verb instead.

It means at the beginning of the sentence or in the beginning of the sentence.

Breakdown:

• in
  • ablative = in / at a place
• initio = ablative singular of initium (beginning)
• sententiae = genitive singular of sententia (sentence)

So:

• in initio = at the beginning
• sententiae = of the sentence

Together:

• in initio sententiae = at the beginning of the sentence

Because in takes the ablative when it means location, and the accusative when it means motion toward something.

Here there is no motion; it means in/at the beginning as a location within the sentence.

So:

• in initio = at the beginning → ablative

Compare:

• in urbem = into the city → accusative, motion toward
• in urbe = in the city → ablative, location

Because it depends on initio and tells us the beginning of what.

• initium = beginning
• initium sententiae = the beginning of the sentence

This is a very common Latin use of the genitive, much like English of.

Saepe means often, and it modifies the verb poni.

So the idea is:

• to be often placed or, in smoother English,
• is often placed

It tells us how frequently the nominative is placed at the beginning of the sentence.

Latin word order is much more flexible than English word order because Latin uses case endings to show grammatical relationships.

English depends heavily on order:

• The teacher says that the nominative is often placed at the beginning of the sentence.

Latin can move words around more freely:

• Magistra dicit nominativum saepe in initio sententiae poni.

The endings already tell you what each word is doing, so the order can be adjusted for emphasis, style, or clarity.

In this sentence:

• Magistra comes first as the main subject.
• dicit comes early to introduce the reported statement.
• nominativum appears before poni, which helps mark it as the subject of the infinitive clause.

Yes, possibly both grammatical and stylistic.

Grammatically, it is helpful to place the accusative subject of the indirect statement before the infinitive.

Stylistically, there may also be a nice bit of self-reference here: the sentence says that the nominative is often placed near the beginning of the sentence, and nominativum itself appears fairly early.

That does not change the grammar, but it is something a teacher or textbook writer might do deliberately.

Here are the main forms:

• magistra, magistrae — female teacher
• dico, dicere, dixi, dictum — say
• nominativus, -a, -um — nominative
  Here the neuter form nominativum is used substantively: the nominative
• saepe — often
• initium, initii — beginning
• sententia, sententiae — sentence
• pono, ponere, posui, positum — place, put
  poni is its present passive infinitive: to be placed

Because Latin normally does not use a separate word equivalent to English that in this kind of indirect statement.

English says:

• The teacher says that the nominative is often placed...

Latin usually says:

• The teacher says the nominative to be often placed...

That sounds unnatural in English, so we translate it with that, but Latin itself expresses the idea through the accusative + infinitive construction instead of a conjunction.

No. Latin has no definite or indefinite articles.

So:

• initio can mean in a beginning, in the beginning, or at the beginning
• sententiae can mean of a sentence or of the sentence

The context tells you what sounds best in English. Here, English naturally uses:

• at the beginning of the sentence

It is the subject of dicit only.

Structure:

• Magistra dicit = The teacher says
• nominativum ... poni = that the nominative is placed

So:

• magistra is the subject of the main verb
• nominativum is the subject of the infinitive clause

That distinction is very important in indirect statement. English speakers often first assume the main subject continues into the subordinate clause, but Latin marks the subordinate subject separately with the accusative.

In some kinds of later or less classical Latin, clauses with quod can sometimes be used after verbs of saying or thinking. But in standard classical Latin, after dicit, the normal construction for reported statements is the accusative and infinitive.

So for a learner, the important point is:

• after dicit, expect accusative + infinitive

That is exactly what you have here:

• nominativum ... poni