Post ientaculum serva calices et patinas lavat, dum domina in atrio epistulam legit.

Because post as a preposition takes the accusative case when it means after.

Here, ientaculum is a neuter second-declension noun. In neuter nouns, the nominative singular and accusative singular are identical, so both are ientaculum. That is why you do not see a different ending here.

So:

• post
  • accusative
• ientaculum = accusative singular form, which happens to look the same as the nominative

In this sentence, serva is the nominative singular, so it is the subject of lavat.

A native English speaker may notice that serva could also look like an ablative singular form in isolation. That is true. But in context, serva is clearly the subject because:

• it fits naturally with the verb lavat = washes
• there is no preposition or other construction suggesting an ablative use
• if serva were the direct object, we would expect servam, not serva

So here serva means the slave-girl / maidservant as the one doing the washing.

Latin does not have articles like English the and a/an.

That means a noun such as serva can mean:

• the slave-girl
• a slave-girl

The exact choice depends on context and translation style. The same is true for domina, calices, patinas, and epistulam.

They are the direct objects because they are the things being washed.

The verb lavat means washes, so we ask: washes what?
Answer: calices et patinas.

Both are in the accusative plural, but they belong to different declensions:

• calix, calicis is a third-declension noun, so its accusative plural is calices
• patina, patinae is a first-declension noun, so its accusative plural is patinas

So the different endings do not mean different jobs in the sentence. They are both direct objects; they just come from different noun patterns.

The ending -t usually marks a third-person singular verb in the present tense.

So:

• lavat = he/she/it washes
• legit = he/she/it reads

Latin usually does not need an expressed subject pronoun like she, because the verb ending already tells you the person and number. The nouns serva and domina make it clear who each she is.

Here dum means while.

It introduces a clause that describes something happening at the same time as the main action:

• main clause: serva calices et patinas lavat
• dum clause: domina in atrio epistulam legit

So dum connects two simultaneous actions: the servant is washing, while the mistress is reading.

This is a very good question, because in unmarked Latin spelling legit can be ambiguous.

Without macrons, legit could represent:

• legit = reads (present)
• lēgit = read / has read (perfect)

In this sentence, the context strongly points to the present:

• dum normally introduces an action going on at the same time
• lavat is clearly present
• the sentence describes an ongoing scene

So here legit should be understood as reads / is reading, not read.

Because in takes different cases depending on meaning:

• in
  • ablative = in / on a place, showing location
• in
  • accusative = into / onto, showing motion toward

Here the mistress is in the atrium, not moving into it. So Latin uses the ablative:

• in atrio = in the atrium

If there were motion into the atrium, Latin would use the accusative: in atrium.

Because it is the direct object of legit.

The verb legit means reads, so we ask: reads what?
Answer: epistulam.

That makes epistulam accusative singular. Its dictionary form is epistula, a first-declension noun, and the accusative singular ending is -am.

Latin shows this mainly through the combination of:

• dum = while
• present-tense verbs: lavat and legit

This creates the sense of two ongoing actions in the same scene:

• the servant is washing
• the mistress is reading

English often uses is washing and is reading to stress ongoing action, but Latin commonly uses the simple present for that idea.

Latin word order is much more flexible than English word order because the endings show each word’s role.

This sentence uses a very natural order:

• time phrase first: Post ientaculum
• subject and objects: serva calices et patinas
• verb: lavat
• then the dum clause

But Latin could rearrange many of these words without changing the basic meaning, for example to emphasize a particular word. The endings would still tell you what is subject, object, or location.

So word order in Latin is important for emphasis and style, but less important for basic grammar than it is in English.