Avia puellam ad pistrinum comitatur.

Because puellam is the direct object of the verb.

The verb comitatur means accompanies or goes with, so the person being accompanied is put in the accusative case. For a first-declension noun like puella, the accusative singular ending is -am:

• puella = nominative singular, the girl as subject
• puellam = accusative singular, the girl as object

So in this sentence, the grandmother is accompanying the girl, which is why Latin uses puellam.

Because avia is the subject of the sentence, not the object.

In Latin, the subject normally appears in the nominative case. For a first-declension noun, the nominative singular ending is usually -a:

• avia = nominative singular, grandmother
• aviam = accusative singular, grandmother as object

So avia is the one doing the action, and puellam is the one receiving it.

Pistrinum is also accusative singular here.

That is because it comes after the preposition ad, which takes the accusative when it means to, toward, or up to a place.

So:

• ad pistrinum = to the bakery / to the mill

This is a very common Latin pattern:

• ad villam = to the house / country house
• ad urbem = to the city
• ad forum = to the forum

They are both in the accusative, but they are doing different jobs.

• puellam is the direct object of comitatur
• ad pistrinum is a prepositional phrase showing direction

So the accusative case is not used only for direct objects. It is also used after certain prepositions, including ad.

Because comitatur is a deponent verb.

A deponent verb has passive-looking forms but an active meaning. So although comitatur looks passive, it actually means:

• he/she accompanies
• not he/she is accompanied

This is one of the important things Latin learners have to get used to. The dictionary form is comitor, comitari, comitatus sum.

So:

• comitatur = he/she accompanies
• it is 3rd person singular present indicative

Very often, yes. With comitor, the person being accompanied is commonly put in the accusative.

So in this sentence:

• avia = the one accompanying
• puellam = the person accompanied
• ad pistrinum = the destination

That gives a very natural structure: Grandmother accompanies the girl to the bakery.

Because Latin word order is much freer than English word order.

English depends heavily on position:

• The grandmother accompanies the girl
  is different from
• The girl accompanies the grandmother

But Latin uses case endings to show who is doing what. That means the sentence could be rearranged without changing the basic meaning, for example:

• Avia puellam ad pistrinum comitatur
• Puellam avia ad pistrinum comitatur
• Ad pistrinum avia puellam comitatur

All of these still mean the same thing, because:

• avia is nominative = subject
• puellam is accusative = object

Word order in Latin often adds emphasis rather than changing the grammar.

Because classical Latin has no articles.

So a noun like avia can mean:

• grandmother
• a grandmother
• the grandmother

The same is true for puellam and pistrinum. The exact choice in English depends on the context.

That means this sentence could be translated in slightly different ways depending on the situation, even though the Latin is the same.

It means to the bakery or to the mill, depending on context.

The noun pistrinum can refer to a place connected with grinding or baking. In many beginner contexts, bakery is the simplest translation, but the broader idea is a place where grain is processed or bread is made.

The important grammar point is:

• ad = to / toward
• pistrinum = place noun in the accusative after ad

Not with the same meaning.

If you want to say to the bakery, Latin normally uses ad with the accusative here:

• ad pistrinum

Without ad, pistrinum by itself would not naturally express motion toward that place in this sentence.

So ad is important because it clearly marks direction.

The verb form comitatur by itself means he/she/it accompanies. Latin verbs in the 3rd person singular do not show gender.

You know it is she here because the subject is avia, and avia is a feminine noun.

So:

• comitatur = he/she/it accompanies
• avia comitatur = grandmother accompanies, therefore she accompanies

It is a second-declension neuter noun.

That matters because neuter second-declension nouns have the same form in the nominative singular and accusative singular:

• pistrinum = nominative singular
• pistrinum = accusative singular

So even though it looks unchanged, here it is accusative because it follows ad.

Yes. Comitatur is present tense, so the sentence means:

• Grandmother accompanies the girl to the bakery
• or in more natural English, possibly Grandmother is accompanying the girl to the bakery

Latin present tense can correspond to either a simple present or a present progressive in English, depending on context.