Postea Marcus dicit se carmen novum de mari componere velle.

Se is the reflexive pronoun in the accusative singular, and here it refers back to Marcus.

So:

• Marcus dicit = Marcus says
• se ... velle = that he wants ...

Latin uses se in this kind of construction when the subject of the reported statement is the same as the subject of the main verb.

So Marcus dicit se ... velle means Marcus says that he wants..., where he = Marcus.

If it referred to someone else, Latin would normally use a different pronoun, not se.

This is a very common Latin pattern called the accusative-and-infinitive construction for indirect statement.

After a verb like dicit (says), Latin often does not use a word like that plus a finite verb. Instead, it uses:

• a subject in the accusative
• a verb in the infinitive

So:

• se velle = that he wants
• se componere velle = that he wants to compose

Here, velle depends on dicit, and componere depends on velle.

You can think of the structure like this:

• Marcus dicit
• se velle
• carmen novum de mari componere

In smoother English: Marcus says that he wants to compose a new song/poem about the sea.

Because this is reported speech after dicit.

If Marcus were speaking directly, Latin could say:

• Marcus vult carmen novum de mari componere.
• Marcus wants to compose a new song/poem about the sea.

But once the sentence becomes Marcus says that..., Latin usually changes the reported verb into an infinitive:

• Marcus dicit se ... velle.

So velle is not random; it is the infinitive form required by the indirect statement construction.

It helps to group the sentence into pieces:

• Postea = afterwards / later
• Marcus dicit = Marcus says
• se ... velle = that he wants
• carmen novum = a new song/poem
• de mari = about the sea
• componere = to compose

So the inner structure is:

• se = subject of the infinitives
• velle = main infinitive of the reported statement
• componere = infinitive complement of velle
• carmen novum = object of componere
• de mari = phrase modifying carmen or the idea of composing it

A useful literal unpacking is:

Later Marcus says himself a new song/poem about the sea to compose to want

That is not good English, but it shows the Latin grammar.

Carmen is in the accusative singular, and novum agrees with it in case, number, and gender.

Why accusative? Because it is the direct object of componere:

• componere quid?
• compose what?
• carmen novum

So:

• carmen = song / poem
• novum = new

Together: a new song/poem

Because the preposition de takes the ablative case.

The noun mare, maris is neuter, and its ablative singular is mari.

So:

• nominative/accusative: mare
• ablative: mari

Therefore:

• de mari = about the sea / concerning the sea

This is a standard preposition + ablative combination.

Carmen can mean several related things in Latin, especially:

• song
• poem
• sometimes verse or chant

Which English word is best depends on context. In a basic learning sentence like this, song or poem could both be reasonable.

So carmen novum de mari could be:

• a new song about the sea
• a new poem about the sea

Grammatically, the subject inside the indirect statement is se.

But se refers back to Marcus, so in meaning the subject is still Marcus.

That means Marcus is the one who wants, and Marcus is also the one who will compose.

So:

• main clause subject: Marcus
• reported clause subject: se
• referent of se: Marcus

This is why English often just says he, but Latin makes the structure explicit with se.

Because Latin usually expresses reported statements differently from English.

English often says:

• Marcus says that he wants...

Latin often says:

• Marcus dicit se velle...

So instead of:

• that
  • finite verb

Latin prefers:

• accusative subject
  • infinitive verb

This is one of the most important patterns for English speakers to get used to in Latin.

Latin word order is more flexible than English word order because the endings show the grammatical relationships.

So this sentence could be rearranged in various ways and still mean the same thing, as long as the forms stay clear. For example, Latin could move postea, Marcus, or de mari for emphasis.

However, the given order is perfectly natural and helpful for reading:

• Postea sets the time
• Marcus dicit gives the main clause
• then the indirect statement follows

So the order is not absolutely fixed, but it is a sensible and common arrangement.

Postea is an adverb meaning afterwards, later, or after that.

It tells you when the action of dicit happens in relation to earlier events in the passage.

So it does not change the grammar of the sentence; it simply adds time information.

Yes. Velle is the infinitive and literally means to want.

But in an indirect statement after dicit, English usually translates it with a finite verb:

• dicit se velle = he says that he wants

So when reading Latin, it is useful to recognize both levels:

• dictionary/basic form: velle = to want
• sentence meaning here: wants

That difference is normal when translating Latin infinitives into natural English.