Post balneum mater ad macellum brassicam, porrum, et rapa emptum it.

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

So:

• post balneum = after the bath

This is just something you learn with the preposition:

• post + accusative

In this sentence, balneum is singular accusative.

Mater is the subject of the sentence, so it is in the nominative case.

Here:

• mater = mother

Even though mater ends in -er, it is nominative singular. Its genitive is matris, so it belongs to the third declension.

Ad means to or toward, and it takes the accusative.

So:

• ad macellum = to the market

This shows motion toward a place. Since the verb is it (goes), Latin uses ad with the accusative to show where someone is going.

Macellum is not just any market in the broadest sense. It usually means a food market or provision market, especially a market where food such as meat, vegetables, and other produce is sold.

So in this sentence, ad macellum suggests going to the kind of market where one would buy vegetables.

They are the things being bought, so they are the direct objects of the idea to buy.

The forms are:

• brassicam = cabbage, accusative singular
• porrum = leek, accusative singular
• rapa = turnips, accusative plural

Even though emptum is not a normal finite verb, it still carries the meaning to buy, and these nouns are its objects.

Because rapa here is neuter plural accusative, not feminine singular accusative.

The noun is:

• rapum, rapi = turnip

Its neuter plural nominative/accusative is:

• rapa

So:

• rapa = turnips

That is why it does not end in -am.

Emptum is a supine in -um, used after a verb of motion to express purpose.

So:

• it emptum = she goes to buy

This is a standard Latin construction:

• verb of motion + supine in -um

Examples:

• venit spectatum = he comes to watch
• it rogatum = he goes to ask
• it emptum = she goes to buy

English usually uses to buy, but Latin here uses the supine instead of an infinitive.

No. Emptum here does not agree with brassicam, porrum, rapa, or anything else.

That is because it is a supine, not an adjective.

It may look like a neuter singular form, but in this sentence it is functioning as a verbal noun used in a special construction after a verb of motion:

• it emptum = goes to buy

So it is best understood as a fixed grammatical form rather than as a word agreeing with another noun.

It is from eo, ire = to go.

So:

• it = he/she/it goes

Latin often uses eo very naturally for simple movement from one place to another. Vadit can also mean goes, but it from eo is extremely common and basic.

In this sentence:

• mater ... it = mother goes

It is present tense, third person singular, indicative active.

So it means:

• she goes or sometimes, depending on context,
• mother is going

Because the subject is mater, we translate it as she goes.

Latin word order is much more flexible than English word order because Latin shows grammatical relationships mainly through endings, not just position.

This sentence is arranged as:

• Post balneum = after the bath
• mater = mother
• ad macellum = to the market
• brassicam, porrum, et rapa = cabbage, leek, and turnips
• emptum = to buy
• it = goes

A very literal order would be:

• After the bath mother to the market cabbage, leek, and turnips to buy goes.

That sounds odd in English, but it is perfectly normal Latin.

Literally, it means after the bath.

But depending on context, English might naturally express that as:

• after the bath
• after bathing
• after her bath

Latin often uses a concrete noun where English might prefer a verbal expression.

Because Latin does not normally use a plain infinitive after a verb of motion to express purpose in this way.

English says:

• She goes to buy vegetables.

But Latin often prefers:

• She goes buy-supine that is,
• it emptum

So the idea is the same, but the grammar is different.

Yes. Latin has other ways to express purpose, but with a verb of motion, the supine in -um is a classic and elegant choice.

So it emptum is a very typical way to say:

• goes to buy

Other purpose constructions exist in Latin, but for this sentence the supine is especially appropriate.

They are:

• post = after
• balneum, balnei = bath
• mater, matris = mother
• ad = to, toward
• macellum, macelli = market, food market
• brassica, brassicae = cabbage
• porrum, porri = leek
• rapum, rapi = turnip
• emo, emere, emi, emptum = buy
• eo, ire, ii/ivi, itum = go

This can help you see where the sentence forms come from:

• emptum comes from emo
• it comes from eo

A useful breakdown is:

• Post balneum = after the bath
• mater = mother
• ad macellum = to the market
• brassicam, porrum, et rapa = cabbage, leek, and turnips
• emptum it = goes to buy

So the structure is:

time expression + subject + destination + things to be bought + motion/purpose expression

That is a good way to understand how the Latin sentence is built.