Pater dicit se tantum solvere non posse.

Because Latin commonly uses indirect statement after verbs like say, think, know, and hear.

The normal pattern is:

verb of saying/thinking + accusative subject + infinitive

So:

• Pater dicit = Father says
• se ... non posse = that he is not able ...

English usually uses that, but Latin usually does not here. Instead, it uses the accusative-and-infinitive construction.

In a Latin indirect statement, the subject is put in the accusative case, not the nominative.

So even though se is the logical subject of posse in English, Latin makes it accusative because that is how the construction works.

Compare:

• direct: non possum = I am not able
• indirect: dicit se non posse = he says that he is not able

So se is not an object here in meaning; it is the subject of the infinitive inside the indirect statement.

Because se is the reflexive pronoun. It refers back to the subject of the main verb, which here is pater.

So:

• Pater dicit se ... = Father says that he himself ...

If Latin used eum, it would usually mean some other man, not the father himself.

This is a very common pattern:

• Marcus dicit se venire = Marcus says that he is coming
• Marcus dicit eum venire = Marcus says that he is coming (but he = someone else)

Because in an indirect statement, Latin uses an infinitive, not a finite verb.

So English that he cannot becomes Latin se ... non posse.

If it were a direct statement, you would expect a finite form such as non potest or non possum, depending on who is speaking. But after dicit, Latin changes the verb into an infinitive.

Because posse means to be able, and it normally takes another infinitive to complete its meaning.

So:

• posse = to be able
• solvere = to pay
• solvere posse = to be able to pay

In this sentence, posse is the main infinitive of the indirect statement, and solvere depends on it.

Tantum is referring to the amount involved. Depending on context, forms like tantum can mean so much, that much, or sometimes only.

Grammatically, it is a neuter singular form being used without an expressed noun, something like:

• tantum [pecuniae]
• that much [money]

So the idea is that the father says he is not able to pay that much / so much, according to the meaning supplied in your translation.

Yes, you can think of an amount as being understood.

Solvere often means to loosen/release, but in financial contexts it commonly means to pay. Here, the amount paid is expressed by tantum, so Latin does not need an extra noun like money unless it wants to be more explicit.

So the sense is basically:

to pay that much

even though a noun such as pecuniam is not stated.

Because non is negating posse: the idea is not to be able.

So the structure is:

• solvere = to pay
• non posse = not to be able

That gives the sense not to be able to pay.

If Latin wanted to express a different idea, such as to be able not to pay, the wording would normally be arranged differently. Here the natural reading is that the inability is what is being negated.

Yes. Latin word order is much freer than English word order.

This sentence is arranged as:

Pater dicit | se tantum solvere non posse

That is perfectly normal. But Latin could also say things like:

• Pater dicit se non posse tantum solvere
• Pater se tantum solvere non posse dicit

with little or no change in basic meaning.

The endings and the construction tell you how the words relate to each other, so Latin does not depend as heavily on word order as English does.

A helpful way to understand it is to imagine the father’s original words.

Directly, he might say:

Tantum solvere non possum.
I cannot pay that much.

When this becomes indirect after dicit, Latin changes it like this:

• non possum → non posse
• I → se (because it refers back to the subject pater)

So:

Pater dicit se tantum solvere non posse.

That is one of the easiest ways to see why the accusative and infinitive appear.