Hilaritas aviae puerum flentem consolatur.

The subject is hilaritas.

You can tell because hilaritas is in the nominative singular, the case normally used for the subject of a finite verb. The verb consolatur is also third person singular, so it matches hilaritas: cheerfulness comforts.

So the basic skeleton is:

• hilaritas = the one doing the action
• consolatur = comforts

Here aviae is most naturally genitive singular, meaning of the grandmother or the grandmother’s.

So hilaritas aviae means the grandmother’s cheerfulness.

Formally, aviae could also be dative singular, since the ending -ae can represent more than one case. But in this sentence, the genitive makes much better sense:

• hilaritas aviae = the grandmother’s cheerfulness
• not likely cheerfulness to/for the grandmother

This is a very common thing in Latin: one ending can be ambiguous, and you decide from grammar and sense.

The most natural reading is that aviae depends on hilaritas, giving the grandmother’s cheerfulness.

Reasons:

• A genitive often modifies a nearby noun.
• hilaritas is an abstract noun, and abstract nouns commonly take a genitive showing whose quality it is.
• the grandmother’s cheerfulness makes very good sense with comforts the crying boy.

Latin word order is flexible, so a genitive does not always have to stand right next to the noun it modifies, but here hilaritas aviae is the clearest pairing.

Puerum is accusative singular of puer, meaning boy.

It is in the accusative because it is the direct object of consolatur. In other words, it is the person being comforted.

So:

• hilaritas = subject
• puerum = direct object

Even though English often relies on word order, Latin relies much more on case endings to show who is doing what to whom.

Because flentem must agree with puerum.

Flentem is the accusative singular masculine form of the present participle of flere, to weep / cry. Since it describes puerum, it has to match it in:

• case: accusative
• number: singular
• gender: masculine

So:

• puerum = accusative singular masculine
• flentem = accusative singular masculine

If the noun were nominative singular masculine, then flens would be the form.

It means the crying boy or the boy who is crying.

Latin often uses a participle where English might use either:

• an adjective-like phrase: the crying boy
• a relative clause: the boy who is crying

So flentem is describing the boy’s current action or state.

Because consolatur comes from a deponent verb: consolor, consolari, consolatus sum.

Deponent verbs:

• have passive-looking forms
• but active meanings

So consolatur looks like it could mean is comforted, but in fact it means comforts.

This is one of the most important things for Latin learners to get used to. With deponent verbs, the form is passive in appearance, but the meaning is active.

It is present tense.

More fully, it is:

• third person singular
• present indicative
• deponent

So it means comforts or is comforting, depending on context.

Because the verb agrees with the subject, not with the total number of words in the sentence.

The subject is only hilaritas, which is singular. The other words are not additional subjects:

• aviae = dependent genitive, of the grandmother
• puerum = direct object
• flentem = participle modifying puerum

So the verb is singular: consolatur.

Yes, the word order is perfectly normal for Latin.

Latin word order is much freer than English because case endings show grammatical function. So Latin does not need to depend so heavily on position.

This sentence is arranged roughly like this:

• Hilaritas aviae = the grandmother’s cheerfulness
• puerum flentem = the crying boy
• consolatur = comforts

A very literal English-like order would be:

• The grandmother’s cheerfulness comforts the crying boy

Latin often places the verb at or near the end, and it often keeps related words grouped in meaningful phrases.

Yes. Since it is a present participle, it can be translated in several natural English ways, depending on style:

• the crying boy
• the weeping boy
• the boy who is crying
• the boy as he cries

The core idea is the same: flentem gives an ongoing action or state associated with puerum.

Yes. Latin, like English, can use an abstract noun as the subject of a sentence.

So hilaritas means cheerfulness, merriment, or good spirits, and it can act as the thing producing the effect:

• the grandmother’s cheerfulness comforts the crying boy

That may sound slightly more literary than everyday English, but it is completely normal grammar.

A helpful way to divide the sentence is:

• hilaritas aviae = the grandmother’s cheerfulness
• puerum flentem = the crying boy
• consolatur = comforts

So the structure is:

• Subject phrase: hilaritas aviae
• Object phrase: puerum flentem
• Verb: consolatur

Seeing those chunks makes the sentence much easier to read.