Post longam moram mater demum domum rediit.

Because post takes the accusative case when it is used as a preposition meaning after.

So:

• post = after
• moram = accusative singular of mora
• longam = accusative singular of longa, agreeing with moram

The whole phrase post longam moram means after a long delay or after a long wait.

Because the adjective has to agree with the noun it describes in:

• gender
• number
• case

Here, mora is:

• feminine
• singular
• accusative, because it follows post

So the adjective must match it:

• longa mora = nominative singular
• longam moram = accusative singular

That is why both words end in -am.

Mater is in the nominative singular, which is the normal case for the subject of a finite verb.

Also, the verb rediit is third person singular, so it matches a singular subject: mater.

So the structure is:

• mater = subject
• rediit = verb

Even though Latin word order is flexible, the endings still show the grammatical roles.

Demum is an adverb. It often means:

• at last
• finally
• only then

In this sentence, it adds the idea that the mother returned only after some delay, or eventually.

So post longam moram and demum are not exactly redundant:

• post longam moram tells you there was a long delay
• demum emphasizes that the action happened finally / only at last

Together they strengthen the sense of delay.

Because Latin often uses the accusative without a preposition for motion toward home.

This is a very common idiom:

• domum ire = to go home
• domum venire = to come home
• domum redire = to return home

So domum here does not mean just any accusative object. It is a special adverbial accusative showing motion toward home.

They are similar, but not identical.

• domum usually means home, in the familiar sense of going to one’s home.
• in domum means into the house/home, with more emphasis on entering the building.

So:

• domum rediit = she returned home
• in domum rediit = she went back into the house

The first is the normal idiomatic expression for homeward motion.

Rediit is perfect tense, third person singular.

It means returned or has returned, depending on context.

From the verb:

• redeo = I go back / return
• redire = to return

So rediit = she returned / he returned / it returned

In this sentence, since the subject is mater, it means the mother returned.

Because redeo is a compound of:

• re- = back
• eo = I go

The verb eo is irregular, and compounds of eo keep some of that irregularity.

So the perfect of redeo is commonly:

• redii or rediit in the third person singular

That double i can look strange at first, but it is normal for this verb family.

Not as much as in English. Latin uses endings more than position to show grammar, so word order is relatively flexible.

This sentence puts the words in a natural and expressive order:

• Post longam moram = sets the time/background first
• mater = introduces the subject
• demum = emphasizes finally
• domum = shows destination
• rediit = ends with the verb

A Roman could rearrange the words and still keep the basic meaning, for example:

• Mater post longam moram demum domum rediit
• Mater domum demum post longam moram rediit

But the original order has a nice flow and emphasis.

No. Redire here is not taking domum as a normal direct object.

Instead, domum is an accusative of place to which in a special idiomatic form. It answers the question where to?, not what?

So:

• not returned the home
• but returned home

This is one of those patterns that English speakers simply have to get used to.

Because home is one of the important exceptions.

Latin often omits a preposition with certain place expressions, especially:

• domum = homeward, home
• names of cities
• names of small islands

For example:

• Romam it = he goes to Rome
• domum venit = he comes home

So domum follows a broader Latin pattern, even though it may feel surprising from an English point of view.

Latin has no articles, so mater by itself can mean:

• the mother
• a mother
• sometimes even mother, depending on context

English has to choose one when translating, but Latin does not mark that distinction with a separate word like the or a.

In this sentence, context would usually make the mother the most natural interpretation.

Yes. Even if the basic meaning is already clear, an English speaker may notice several natural possibilities:

• after a long delay
• after a long time
• after a long wait

The Latin noun mora often carries the idea of delay or lingering, so after a long delay is often the closest grammatical equivalent, but English may phrase it differently depending on style and context.