Hodierna laetitia etiam avum, qui saepe tacet, ad risum movet.

Because hodierna is an adjective modifying laetitia.

• laetitia = joy, happiness
• It is feminine, singular, nominative here.
• So the adjective has to match it in gender, number, and case.

That is why we get:

• hodierna laetitia = today’s joy or the joy of today

So hodierna is not standing alone; it is agreeing with laetitia.

Laetitia is in the nominative singular, and it is the subject of movet.

A good way to see this is through the verb:

• movet = moves
• The thing doing the moving is hodierna laetitia
• The thing being moved is avum

So the structure is:

• hodierna laetitia = subject
• avum = direct object
• movet = verb

Even though the word order is not the same as English, the cases tell you the roles.

Because avum is the direct object of movet.

The verb moveo, movere often takes a direct object: someone or something is moved, stirred, affected, prompted.

Here:

• laetitia movet avum = joy moves grandfather

So avum is accusative because it is the person affected by the action.

Also note:

• avus = grandfather
• avum = grandfather as direct object

This is a very common question.

In Latin, a relative pronoun agrees with its antecedent in:

• gender
• number

But its case depends on its role inside its own clause.

Here:

• antecedent: avum = masculine singular
• relative pronoun: qui = masculine singular nominative

Why nominative? Because in the relative clause qui saepe tacet, the pronoun is the subject of tacet.

So:

• avum is accusative in the main clause
• qui is nominative in the relative clause

That is completely normal in Latin.

It is a relative clause describing avum.

Breakdown:

• qui = who
• saepe = often
• tacet = is silent, keeps quiet

So the clause means:

• who is often silent
• or who often keeps quiet

This clause gives extra information about avum. It tells us what kind of grandfather we are talking about, or adds a descriptive detail about him.

Tacet is:

• 3rd person singular
• present
• active
• indicative

from the verb taceo, tacere = to be silent, to keep quiet

So tacet literally means:

• he/she/it is silent
• he/she/it keeps quiet

Because the subject in this clause is qui referring to avus, we understand it as:

• he is often silent
• who often keeps quiet

Etiam usually means also, even, or still, depending on context.

Here it most naturally means:

• also
• or even

Its placement before avum suggests emphasis on avum:

• even grandfather
• or grandfather too

So the sentence gives the feeling that today’s joy affects not only others, but even grandfather, despite the fact that he is often silent.

Latin word order is flexible, and adverbs like etiam are often placed near the word they especially emphasize.

This is an idiomatic Latin construction.

• ad
  • accusative often means to, toward, or into
• risum is the accusative of risus = laughter

So:

• ad risum movere = to move to laughter
• more natural English: to make someone laugh or to move someone to laughter

This is similar to other Latin expressions where ad + accusative shows the result or goal of an action.

So avum ad risum movet means:

• it moves grandfather to laughter
• that is, it makes grandfather laugh

Because it follows the preposition ad, and ad takes the accusative case.

So:

• ad
  • accusative = to, toward
• risus is the noun
• risum is its accusative singular form

That is why we get ad risum.

Because its subject is singular:

• hodierna laetitia = today’s joy

That whole phrase is grammatically singular, so the verb must also be singular:

• movet = moves

If the subject were plural, the verb would be plural too.

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

So a Latin noun like avum can mean, depending on context:

• a grandfather
• the grandfather
• simply grandfather

Likewise laetitia can be:

• joy
• the joy
• happiness

English has to choose an article when translating, but Latin usually leaves that to context.

For an English speaker, yes, it may feel unusual, but it is perfectly normal Latin.

A more mechanically arranged version would be something like:

• Hodierna laetitia movet etiam avum ad risum.
• with the relative clause added: Hodierna laetitia movet etiam avum, qui saepe tacet, ad risum.

But Latin often uses word order for emphasis and style, not just grammar.

In the original sentence:

• Hodierna laetitia comes first, setting the scene
• etiam avum highlights that even grandfather is affected
• ad risum movet saves the key verb until the end, which is very common in Latin prose

So the order is not random; it helps shape emphasis.

In modern printed Latin, the commas help show that qui saepe tacet is an inserted descriptive clause.

So the commas tell the reader:

• this is extra information about avum

In ancient manuscripts, punctuation was much less standardized, so you should not think of the commas as essential to the grammar itself. The grammar comes from the forms:

• qui introduces a relative clause
• tacet gives its verb

The commas are mainly a reading aid in modern editions.

Not by itself in this sentence. Hodierna is an adjective, not an adverb.

So here it means:

• today’s
• of today
• present-day in some contexts

Because it modifies laetitia, the phrase means:

• today’s joy

If Latin wanted the adverb today, it would normally use hodie.

So compare:

• hodie = today
• hodierna laetitia = today’s joy

Yes. The dictionary form is:

• avus, -i = grandfather

In the sentence we see:

• avum = accusative singular

So if you are looking it up, you should look for avus, not avum.

This is a good general habit in Latin: nouns are usually listed in the nominative singular in dictionaries.

Yes. A very literal translation is helpful for grammar, but a more natural English translation may be better stylistically.

Literal:

• moves [him] to laughter

More natural:

• makes him laugh
• brings him to laughter
• stirs him to laughter

So once you understand the grammar, it is perfectly fine to translate more idiomatically into good English.