Cicada in arbore canit, et pueri eam audiunt.

Because the preposition in takes the ablative case when it means in/on in the sense of location.

• arbor = tree (dictionary form, nominative)
• arbore = in the tree / on the tree (ablative singular)

So in arbore means in the tree or on the tree, depending on context.

It is in the ablative because it follows in with a location meaning.

A useful rule is:

• in + ablative = in/on somewhere
• in + accusative = into/onto somewhere

So:

• in arbore = in/on the tree
• in arborem = into the tree or onto the tree

Here the cicada is already there, so Latin uses in arbore.

Because cicada is in the nominative singular, which is the normal case for the subject of a verb.

Also, canit is third person singular, so it matches a singular subject:

• cicada = singular subject
• canit = sings, singular verb

Latin often shows the subject through the noun’s ending and the verb’s ending together.

Canit comes from the verb cano, canere, meaning to sing.

Here:

• canit = he/she/it sings

Since the subject is cicada, the meaning is the cicada sings.

The ending -it shows third person singular present tense.

Because pueri is nominative plural, and audiunt is third person plural.

So they match:

• pueri = boys, nominative plural
• audiunt = they hear, plural verb

That tells you the boys are the subject of the second clause.

Eam is the accusative singular feminine form of the pronoun meaning her or it.

It refers back to cicada, which is a feminine noun in Latin.

So:

• cicada = feminine singular
• eam = her/it in the accusative singular

Since audiunt needs a direct object, Latin uses eam for the boys hear it.

Because Latin pronouns usually agree with the grammatical gender of the noun they refer to, not with natural English habits.

Cicada is a feminine noun, so the pronoun must also be feminine:

• nominative: ea = she/it
• accusative: eam = her/it

Even though in English we would usually say it, Latin still uses the feminine form because cicada is grammatically feminine.

Eam is in the accusative case because it is the direct object of audiunt.

Ask the question:

• The boys hear whom/what?

Answer:

• eam

That is why the accusative is used.

Classical Latin has no definite article and no indefinite article.

So Latin does not have separate words for:

• the
• a
• an

That means:

• cicada can mean a cicada or the cicada
• arbore can mean a tree or the tree
• pueri can mean boys or the boys

The exact sense comes from context.

Yes. Latin word order is much freer than English word order because the endings show the grammatical roles.

So these would still mean essentially the same thing:

• Cicada in arbore canit, et pueri eam audiunt.
• In arbore cicada canit, et pueri eam audiunt.
• Pueri eam audiunt, et cicada in arbore canit.

However, changing the order can shift emphasis or style.

For example, putting in arbore first may emphasize the location.

Et means and. It joins the two clauses:

• Cicada in arbore canit
• pueri eam audiunt

So it connects two related actions:

• the cicada sings
• the boys hear it

Both verbs are in the present tense.

• canit = sings / is singing
• audiunt = hear / are hearing

Latin present tense can often be translated in more than one natural English way, depending on context.

The ending -unt is a common sign of the third person plural present active.

So audiunt means they hear.

That matches pueri, which is plural.

Compare:

• audit = he/she/it hears
• audiunt = they hear

Either can be possible, depending on the situation.

Latin in with the ablative often covers meanings that English separates into in and on. With a cicada, English might naturally say:

• in the tree
• on the tree

Both are reasonable, and context would decide which sounds best.

Yes, it could have repeated the noun, but using eam is more natural once the cicada has already been mentioned.

Compare:

• et pueri eam audiunt = and the boys hear it
• et pueri cicadam audiunt = and the boys hear the cicada

The pronoun avoids repetition, just as English often does.

Because pueri can have more than one form-based meaning, but the sentence structure makes the meaning clear here.

Pueri can be:

• nominative plural = boys
• genitive singular = of the boy

But here it goes with audiunt, which is plural, so pueri must be the plural subject:

• the boys hear

It cannot mean of the boy in this sentence.