Canistrum a serva portatum in mensa positum est.

Because the sentence is passive.

In an active sentence, the thing receiving the action would usually be in the accusative. But in a passive sentence, the thing receiving the action becomes the subject, so it goes into the nominative.

So here:

• canistrum = the subject, the basket
• it is the thing that was placed

That is why canistrum is nominative, not accusative.

They all have neuter singular forms here.

• canistrum is a neuter singular noun
• portatum is a perfect passive participle
• positum is also a perfect passive participle

The participles portatum and positum must agree with canistrum in:

• gender: neuter
• number: singular
• case: nominative

So they also appear as -um.

This is a very common feature of Latin: adjectives and participles match the noun they describe.

A serva gives the agent of a passive action: by the slave woman or by the maidservant.

In Latin, when a passive verb has a personal agent, Latin normally uses:

• a or ab
  • ablative

So here:

• serva is ablative singular of serva
• a serva = by the slave woman

This tells you who carried the basket.

Both a and ab mean the same thing here.

A common rule taught to beginners is:

• ab before a vowel or sometimes for clarity
• a before a consonant

Since serva begins with a consonant, a serva is completely normal.

So this is mostly a matter of normal sound and style, not a change in meaning.

Portatum is a perfect passive participle meaning having been carried or simply carried.

Here it modifies canistrum:

• canistrum a serva portatum = the basket carried by the slave woman

So portatum is not the main verb of the sentence. It is part of a participial phrase describing the basket.

The main verb is:

• positum est = was placed or has been placed

A natural way to understand the whole structure is:

• The basket, carried by the slave woman, was placed on the table
• or The basket, after being carried by the slave woman, was placed on the table

Because only positum est is functioning as the main finite verb.

• portatum = a participle describing canistrum
• positum est = the main verb, perfect passive

So the structure is roughly:

• subject: canistrum
• description of subject: a serva portatum
• main verb: in mensa positum est

English often prefers to turn this kind of Latin participial phrase into a relative clause or another full clause:

• The basket, which was carried by the slave woman, was placed on the table
• After the slave woman carried the basket, it was placed on the table

Positum est is perfect passive indicative, third person singular.

It is formed from:

• positum = perfect passive participle
• est = present tense of sum

Together they make the perfect passive.

Depending on context, Latin perfect can often be translated as either:

• was placed
• has been placed

Both are possible English renderings of the Latin perfect, so the exact choice depends on context and style.

Here it means on the table.

With in + ablative, Latin often expresses location. In English, the best translation may be:

• in
• on
• sometimes even at

depending on the noun and the situation.

Since a basket is placed on top of a table, in mensa is naturally understood as on the table.

So:

• in mensa = on the table

Because it follows in in the sense of location.

Latin uses:

• in
  • ablative for position or location: in/on
• in
  • accusative for motion into: into

So here:

• in mensa = on the table, location
• not movement into the table

If the phrase meant motion into something, Latin would use the accusative instead.

It is describing the basket.

Because portatum agrees with canistrum, it attaches to that noun and tells you something about it:

• the basket carried by the slave woman

Then positum est tells what happened to that basket:

• was placed on the table

So the sentence has one main event, being placed, while being carried is presented as prior or background information about the basket.

Yes, that is the natural sense.

The participle portatum presents the basket as already carried when the main action positum est happens. So the sequence is understood as:

1. the slave woman carried the basket
2. the basket was placed on the table

Latin does not always spell out after explicitly, but the participial structure naturally suggests that earlier action.

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

So even though English strongly prefers something like:

• The basket carried by the slave woman was placed on the table

Latin can arrange the same elements more freely:

• Canistrum a serva portatum in mensa positum est

A learner-friendly way to see the structure is:

• Canistrum = subject
• a serva portatum = description of the subject
• in mensa = where
• positum est = main verb at the end

Placing the verb at the end is especially common in Latin.