Pater quaerit quantum pretium bibliopola pro illo volumine poscat.

Because quantum pretium bibliopola pro illo volumine poscat is an indirect question after quaerit.

A direct question would be:

• Quantum pretium bibliopola pro illo volumine poscit?
  = How much is the bookseller asking for that book/scroll?

When Latin turns that into an indirect question after a verb like quaerit (asks, inquires), it normally uses the subjunctive:

• Pater quaerit ... poscat
  = Father asks how much the bookseller is asking...

So poscat is subjunctive because it is inside an indirect question, not because the action is doubtful or unreal.

The indirect question is:

• quantum pretium bibliopola pro illo volumine poscat

It depends on quaerit.

You can think of the sentence as:

• Pater quaerit = Father asks / inquires
• quantum pretium ... poscat = how much price the bookseller is asking

In smoother English: Father asks how much the bookseller is asking for that volume.

The word that signals the question is quantum.

Quantum here is an interrogative adjective meaning how great a or, more naturally here, how much. It modifies pretium.

Since pretium is:

• neuter
• singular
• accusative

the adjective must match it, so we get:

• quantum pretium

not quanta pretium or quantus pretium.

So literally this part means:

• how much price

But in English we usually just say:

• how much

or

• what price

Pretium is accusative singular.

That is because it is the object of poscat. The bookseller is asking/demanding a price, so the thing being asked is in the accusative.

So:

• bibliopola = subject of poscat
• pretium = object of poscat

A very literal structure is:

• the bookseller asks/demands how much price

Because bibliopola is the subject of poscat.

Latin word order is much freer than English word order, so the subject does not have to come first. Its job is shown by its case, not by position.

Here:

• bibliopola is nominative singular
• therefore it is the subject: the bookseller

Even though English strongly prefers the bookseller asks, Latin can say:

• quantum pretium bibliopola ... poscat

without causing confusion.

Because bibliopola is one of those Latin nouns of Greek origin that belong to the first declension but refer to a male person.

So it looks like a typical first-declension noun, but it is masculine in meaning.

Its nominative singular is:

• bibliopola = bookseller

This is similar to other masculine first-declension nouns such as:

• poeta = poet
• agricola = farmer
• nauta = sailor

So the -a ending does not automatically mean feminine.

Volumine is ablative singular, because it follows the preposition pro.

Here pro means something like:

• for
• in exchange for
• as the price for

So:

• pro illo volumine = for that volume

And:

• illo is also ablative singular, agreeing with volumine

So the phrase means for that particular volume/book/scroll.

Ille, illa, illud is the common demonstrative meaning that.

Here:

• illo = ablative singular masculine or neuter
• it agrees with volumine, which is neuter ablative singular

So illo volumine means that volume.

Using ille often points something out more clearly, almost like that one there. In many contexts, English simply translates it as that.

Volumen originally means a roll or scroll, something rolled up. By extension it can also mean a book or volume.

So here it likely means:

• volume
• book
• or, depending on context, scroll

It is not exactly the same word as liber, though both can be translated as book in many contexts.

Very roughly:

• liber = book
• volumen = roll/scroll, and later also a volume

So the sentence may have a slightly more specific feel than simply book.

Yes, but they are different kinds of asking.

• quaerit means asks, inquires, tries to find out
• poscat comes from poscere, which means to ask for, demand, or request

So:

• Pater quaerit = Father is asking / finding out
• bibliopola ... poscat = the bookseller is asking for / charging

A natural English paraphrase would be:

• Father asks what price the bookseller is charging for that volume.

So the two verbs are not redundant; they describe two different actions.

Because Latin often uses poscere (to ask for, demand) where English might use:

• ask
• charge
• demand

In the context of a seller and a price, poscere pretium can naturally mean:

• to ask a price
• to charge a price

So even though the literal sense is ask for, the idiomatic English meaning may be charge.

The direct question would be:

• Quantum pretium bibliopola pro illo volumine poscit?

That means:

• How much is the bookseller asking for that volume?

Notice the change:

• direct question: poscit (indicative)
• indirect question after quaerit: poscat (subjunctive)

That is one of the most important grammar points in the sentence.

Yes. It is perfectly normal Latin word order, even if it feels less natural in English.

Latin often places the main verb early or late, and it can place the subject after the object inside a subordinate clause without any problem.

This sentence is arranged as:

• Pater quaerit
• quantum pretium
• bibliopola
• pro illo volumine
• poscat

English would usually prefer something more like:

• Father asks how much the bookseller is asking for that volume.

Latin relies much more on endings and much less on position.

It can, depending on context, because quaerere has a range of meanings such as:

• seek
• ask
• inquire
• investigate

But with an indirect question like this, quaerit is very naturally understood as:

• asks
• inquires
• tries to find out

So in this sentence, Father is finding out how much the bookseller is asking is a good sense.

Because that is not how the sentence is structured.

The father is not directly asking the bookseller in the grammar of this sentence. Instead, he is asking/inquiring how much the bookseller is asking.

So bibliopola belongs with poscat, not with quaerit.

The structure is not:

• Father asks the bookseller...

but rather:

• Father asks how much the bookseller asks...

That is why bibliopola is nominative as the subject of poscat, not some object or ablative after quaerit.