Puer canistrum plēnum fert; in eō catīnus parvus et duo mala sunt.

Because puer is in the nominative singular, which is the normal case for the subject of a Latin sentence.

• puer = the boy
• fert = carries

So Puer ... fert means The boy carries ...

Latin often shows grammatical roles by case endings rather than by word order.

Canistrum is the direct object of fert, so it appears in the accusative singular.

• canistrum = basket
• The boy is carrying the basket, so basket is what receives the action.

This form happens to look the same as the nominative for many neuter second-declension nouns:

• nominative singular: canistrum
• accusative singular: canistrum

Because plēnum is an adjective agreeing with canistrum.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• canistrum is neuter
• singular
• accusative

So the adjective must also be neuter singular accusative:

• canistrum plēnum = a full basket / the full basket

Here it simply means full.

So canistrum plēnum means a full basket or the basket, full.

Sometimes Latin can also use plēnus with another word to show what something is full of, but that is not happening here. In this sentence, it just describes the basket as full.

The semicolon separates two closely related statements:

• Puer canistrum plēnum fert = The boy carries a full basket
• in eō catīnus parvus et duo mala sunt = In it are a small bowl and two apples

So the second part explains what is in the basket.

In Latin, punctuation is often added by modern editors to help readers. It is useful here, but it is not itself a grammar feature of Latin.

Because in with the ablative often means in or inside when showing location.

• in = in
• eō = it in the ablative singular

So:

• in eō = in it

Here eō refers back to canistrum, which is a neuter singular noun.

Because after in meaning in/inside, Latin uses the ablative for location.

The pronoun is, ea, id changes form by case:

• nominative/accusative neuter singular: id
• ablative neuter singular: eō

Since the meaning is in it, not it by itself, Latin needs the ablative:

• in eō = in it

If you used id, that would be the wrong case here.

Because the true subject of sunt is not just catīnus parvus. It is the whole compound subject:

• catīnus parvus
• et duo mala

Together they mean:

• a small bowl and two apples

That is a plural subject overall, so the verb is plural:

• sunt = are

Mala is the nominative plural of malum, meaning apple.

• singular: malum = apple
• plural: mala = apples

This is a neuter second-declension noun, and neuter plurals in this declension usually end in -a.

So:

• duo mala = two apples

Because Latin numerals also agree with the nouns they modify.

Duo is the masculine/neuter nominative plural form of two. Since mala is neuter plural, the correct form is duo.

Compare:

• masculine: duo
• feminine: duae
• neuter: duo

So:

• duo mala = two apples

Because it is part of the subject of sunt.

In the second clause, Latin is saying that certain things are in the basket:

• catīnus parvus = a small bowl
• duo mala = two apples

Since these are the things that are there, they are in the nominative.

Latin word order is much freer than English word order because Latin uses case endings to show grammatical function.

English depends heavily on position:

• The boy carries the basket

Latin can move words around more easily:

• Puer canistrum plēnum fert
• Canistrum plēnum puer fert
• and so on

The basic meaning stays clear because:

• puer is nominative
• canistrum is accusative
• plēnum agrees with canistrum
• sunt is plural
• in eō shows location

The chosen order often helps with style, emphasis, or flow, not just bare grammar.

It is neuter here, because it refers back to canistrum, and canistrum is a neuter noun.

The form eō can be masculine or neuter in the ablative singular, so you decide from the noun it refers to. Since the antecedent is canistrum, the meaning here is:

• in eō = in it, referring to the basket

Yes. Latin could place it in different positions, for example:

• in eō catīnus parvus et duo mala sunt
• catīnus parvus et duo mala in eō sunt

Both are grammatical. The version in your sentence puts in eō early, which immediately tells the reader the location: in it.

The second clause follows a common Latin pattern:

• in eō = location
• catīnus parvus et duo mala = subject
• sunt = verb

Literally, it is something like:

• In it a small bowl and two apples are

More natural English:

• In it are a small bowl and two apples
• or There is a small bowl and two apples in it

Latin often places the verb sunt at the end of the clause.