Canistrum, in quo multus panis est, serva ad mensam fert.

Because canistrum is the direct object of fert (carries).

• canistrum = basket
• fert = she carries

So the sentence is about the slave-girl carrying the basket. In Latin, the direct object usually goes in the accusative case, and canistrum is the accusative singular form.

Because canistrum is a neuter second-declension noun, its nominative singular and accusative singular look the same. So you identify its function from the sentence, not just from the form by itself.

Serva is the nominative singular form, so it is the subject of fert.

• serva = slave-girl / maidservant
• nominative singular = the form typically used for the subject

So:

• serva fert = the slave-girl carries

An English speaker may expect the subject to come first, but Latin does not need to do that. The endings tell you the grammar more clearly than the position does.

Because quo is the form required by the preposition in here.

The phrase in quo means in which.

Here is what is happening:

• the antecedent is canistrum (basket), which is neuter singular
• the relative pronoun must agree with its antecedent in gender and number
• but its case depends on its role inside the relative clause

Inside the clause, the meaning is in which there is much bread, so quo is:

• neuter singular
• ablative, because in meaning in or inside takes the ablative

So quod would be wrong here, because that would be nominative or accusative, not the case needed after in.

It is a relative clause describing canistrum.

Break it down:

• in quo = in which
• multus panis = much bread
• est = is

So the clause literally means:

• in which much bread is

More natural English would be:

• in which there is much bread

This clause gives extra information about the basket.

Because panis here means bread as a substance, not loaves or pieces of bread.

Latin often uses panis in the singular as a kind of mass noun, much like English bread.

So:

• multus panis = much bread

The adjective multus agrees with panis:

• panis is masculine singular
• so the adjective is also masculine singular: multus

If you wanted many loaves or many breads in a countable sense, Latin would phrase that differently.

Because its subject is multus panis, which is singular.

Even though English much bread may feel like a large quantity, grammatically it is still singular:

• multus panis est = much bread is / more naturally there is much bread

So the verb is singular: est, not sunt.

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

English depends heavily on position:

• The slave-girl carries the basket is different from
• The basket carries the slave-girl

Latin, however, can show subject and object by form:

• serva = nominative subject
• canistrum = accusative object

So the author can place words in a different order for emphasis or style. In this sentence:

• Canistrum comes first, so the basket is highlighted first
• the relative clause is inserted right after it to describe it
• fert comes at the end, which is very common in Latin

Mensam is accusative singular because it follows ad.

• ad takes the accusative
• ad mensam means to the table

This shows motion toward something.

Compare:

• ad mensam = to the table
• in mensa = on/in the table would be a different construction and meaning

The commas show that this is an inserted descriptive clause about canistrum.

So the main sentence is:

• Canistrum ... serva ad mensam fert.
• The slave-girl carries the basket to the table.

And the relative clause adds extra information:

• in quo multus panis est
• in which there is much bread

In modern printed Latin, punctuation helps the reader. Classical Latin manuscripts originally had much less punctuation, so the commas are an editorial aid, not an ancient requirement.

Yes, a beginner might notice that serva! can also be the imperative of servare (save!, preserve!). But here it is clearly the noun serva.

Why?

Because the sentence structure fits the noun perfectly:

• serva ... fert = the slave-girl carries

If serva were a verb, the sentence would not make good grammatical sense in this context.

So here serva is definitely the noun, nominative singular.

Usually yes, but in smoother English you may translate it more naturally depending on context.

Here:

• canistrum, in quo multus panis est
  literally: the basket, in which much bread is
• more natural English: the basket, in which there is much bread
• or even: the basket that has a lot of bread in it

So in quo literally means in which, but good English may reshape it a little.

The main clause is:

• Canistrum serva ad mensam fert.
• The slave-girl carries the basket to the table.

The subordinate clause is the relative clause:

• in quo multus panis est
• in which there is much bread

This subordinate clause depends on canistrum and describes it.

So the sentence structure is:

• object
• relative clause describing the object
• subject
• prepositional phrase
• verb