Canistrum panis plenum est, et serva id ad mensam fert.

That is a very common question. Canistrum is a neuter second-declension noun, and in neuter nouns the nominative singular and accusative singular look the same.

So:

• nominative singular: canistrum
• accusative singular: canistrum

In this sentence, canistrum is the subject because it goes with est: canistrum ... est = the basket is ...

Here panis is best understood as genitive singular, meaning of bread.

That can be confusing because panis has the same form for:

• nominative singular: panis
• genitive singular: panis

In this sentence, the sense is full of bread, and Latin often uses the genitive with plenus (full). So panis plenum means full of bread.

Because plenum agrees with canistrum.

Canistrum is:

• neuter
• singular
• nominative

So the adjective must match it:

• masculine: plenus
• feminine: plena
• neuter: plenum

Since the basket is neuter, Latin says canistrum plenum.

Plenum est means is full.

This is a very standard Latin pattern:

• canistrum = the basket
• est = is
• plenum = full

So plenum is a predicate adjective: it describes the subject through the verb est.

Serva is nominative singular, and it is the subject of fert.

So:

• serva = the female slave / maidservant
• fert = carries

Together: serva ... fert = the servant carries ...

Id means it, and it refers back to canistrum.

Since canistrum is neuter singular, the pronoun must also be neuter singular:

• masculine: eum
• feminine: eam
• neuter: id

Here id is the direct object of fert: the servant carries it.

Yes, often it could.

Latin frequently leaves out words that are clear from context. So a sentence without id could still be understood. But including id makes the reference explicit: the servant is carrying the basket.

Because ad means to or toward, and it takes the accusative case.

So:

• mensa = table
• mensam = accusative singular
• ad mensam = to the table

This shows movement toward a destination.

Fert is third-person singular present active indicative of the irregular verb ferre, meaning to carry or to bear.

So fert means:

• he carries
• she carries
• it carries

Here, because the subject is serva, it means she carries.

Because Latin word order is much more flexible than English word order. Latin relies heavily on case endings instead of position in the sentence.

So even though English prefers something like:

The basket is full of bread, and the servant carries it to the table,

Latin can say:

Canistrum panis plenum est, et serva id ad mensam fert.

The endings tell you which word is the subject, object, and so on.

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

So a word like canistrum can mean:

• a basket
• the basket

The exact sense depends on context. The same is true for serva and mensa.