In horto puer malum rubrum videt et folia viridia sub arbore.

Because in takes the ablative when it means in/on/at a place where something already is.

So:

• in horto = in the garden
• but in hortum would mean into the garden

This is a very common Latin pattern:

• in + ablative = location
• in + accusative = motion toward

Here the boy is not moving into the garden; he is simply there, so horto is ablative.

For the same basic reason: sub can take either the ablative or the accusative, depending on meaning.

• sub arbore = under the tree (location)
• sub arborem = to a position under the tree (motion toward)

Since this sentence describes where the leaves are, Latin uses the ablative: arbore.

Because puer is in the nominative singular, which is the normal case for the subject.

Also, the verb videt is third person singular, so it matches a singular subject:

• puer = nominative singular
• videt = he/she/it sees

Even though Latin word order is flexible, the case ending helps you identify the job of the word.

Because they are in the accusative case, which here marks the direct objects of videt.

The boy is seeing something, and the things seen are:

• malum rubrum
• folia viridia

So:

• puer = subject
• videt = verb
• malum rubrum et folia viridia = direct objects

Because the adjective rubrum has to agree with malum in:

• gender
• number
• case

Here:

• malum is neuter singular accusative
• so rubrum must also be neuter singular accusative

This is called adjective agreement.

A useful idea for English speakers: Latin adjectives change form to match the noun, much more than English adjectives do.

Again, adjective agreement.

• folia is neuter plural accusative
• so viridia must also be neuter plural accusative

That is why Latin uses viridia, not virides.

This is especially important with neuter plural nouns, because English does not show this kind of agreement clearly.

Folia is the plural form meaning leaves.

Its singular is:

• folium = leaf
• folia = leaves

This is a neuter second-declension noun, so the neuter plural nominative/accusative ending is -a.

That can feel strange to English speakers because -a often looks singular, but in Latin neuter nouns it is often plural.

Because Classical Latin has no articles.

So a noun like puer can mean, depending on context:

• the boy
• a boy
• simply boy in a general sense

The same is true for malum, folia, arbore, and so on.
English must choose the or a/an, but Latin usually leaves that to context.

Latin word order is much freer than English word order because the endings show each word’s role.

So Latin can say:

• In horto puer malum rubrum videt et folia viridia sub arbore

without confusion, because:

• puer looks like a subject
• malum and folia look like objects
• horto and arbore look like ablatives

That said, word order still affects emphasis. Starting with In horto sets the scene first: in the garden.

Most naturally, it goes with the nearest phrase, folia viridia:

• the boy sees a red apple
• and green leaves under a tree

So readers will usually take sub arbore as describing where the leaves are.

However, because Latin word order is flexible, context can matter. If a writer wanted to make the attachment absolutely clearer, they could rewrite the sentence.

Videt is from the verb vidēre, meaning to see.

Its form here is:

• present tense
• active voice
• third person singular

So it means:

• he sees
• she sees
• it sees

Since the subject is puer, we understand it as the boy sees.

Because arbor is a third-declension noun, and third-declension nouns do not all form their ablative singular the same way second-declension nouns do.

Its main forms are:

• arbor = nominative singular
• arborem = accusative singular
• arbore = ablative singular

So sub arbore is exactly the expected ablative form of arbor.

Yes. Malum can also mean evil in other contexts.

That is because Latin has two different words that happen to look the same in some forms:

• mālum = apple
• malum = evil

In normal printed Latin for learners, vowel length is often not marked, so context tells you which meaning is intended. Here, because of rubrum and the overall scene, malum clearly means apple.