De professor vraagt altijd uit welke bron een citaat komt.

Because de professor is third-person singular: he/she asks.

In the present tense, the verb vragen changes like this:

• ik vraag — I ask
• jij vraagt / vraag je — you ask
• hij/zij vraagt — he/she asks
• wij/jullie/zij vragen — we/you/they ask

So with de professor, you need vraagt.

Dutch main clauses normally follow the V2 rule: the finite verb goes in the second position.

So in:

De professor vraagt altijd ...

• De professor = first element
• vraagt = finite verb in second position
• altijd = adverb after the verb

This is very normal Dutch word order.

You could move things around for emphasis, but then the verb still stays second:

• Altijd vraagt de professor ... — possible, but marked/emphatic

The most neutral version is the original one.

Uit means out of / from, and with bron it is the normal preposition for expressing the source something comes from.

So uit welke bron means from which source.

Dutch often uses uit when talking about the origin of information, text, or material:

• uit een boek — from a book
• uit een artikel — from an article
• uit welke bron — from which source

English learners sometimes expect van, because English uses from very broadly, but in Dutch the choice of preposition depends on the noun and expression.

Because welke is used when you are choosing or asking about which specific one from a known or possible set.

• welke bron = which source
• wat = what

So:

• uit welke bron = from which source
• wat voor bron = what kind of source

The sentence is asking for identification of the source, so welke is the right choice.

Een citaat means a quotation, not the quotation.

Dutch uses:

• een for a/an (indefinite)
• de/het for the (definite)

So the sentence refers to a quotation in a general sense, not one specific quotation already known in the conversation.

If you wanted to refer to a specific quotation, you could say:

• ... uit welke bron het citaat komt.

That would mean which source the quotation comes from.

Because uit welke bron een citaat komt is an embedded question (also called an indirect question).

In Dutch, the conjugated verb usually goes to the end in subordinate clauses and embedded questions.

So:

• direct question: Uit welke bron komt een citaat?
• embedded question: De professor vraagt ... uit welke bron een citaat komt.

This is one of the most important word-order patterns in Dutch.

Yes. It is an indirect question.

The whole sentence is a statement:

• De professor vraagt altijd ... — The professor always asks ...

What follows is the content of that question:

• uit welke bron een citaat komt — which source a quotation comes from

Because the full sentence is not being asked directly to the listener, it ends with a period, not a question mark.

Yes, in some contexts that is possible.

Waaruit can mean from what / from which thing. But there is a difference in feel:

• uit welke bron is more explicit and formal: from which source
• waaruit is shorter and more general: from what

Because the sentence includes the noun bron, it sounds more precise and academic, which fits well with de professor and een citaat.

Because professor is a de-word.

In Dutch, every noun has a grammatical article:

• de professor
• het citaat
• de bron

Unfortunately, this often has to be learned noun by noun. There are some patterns, but for many nouns you simply memorize the article with the word.

A good habit is to learn them together:

• de professor
• de bron
• het citaat

The sentence uses the present tense because it describes something that usually happens:

• vraagt altijd = always asks

Dutch, like English, often uses the present tense for habitual actions.

So the meaning is not about one single event in the past. It describes a regular behavior of the professor.

It breaks down like this:

• De professor = subject
• vraagt = finite verb
• altijd = adverb
• uit welke bron een citaat komt = embedded question, functioning as the object/content of vraagt

So the professor is asking something, and that something is the indirect question: from which source a quotation comes.

Yes. The direct question would be:

Uit welke bron komt een citaat?

Here, because it is a direct question, the finite verb komt comes before the subject een citaat.

Compare:

• Direct question: Uit welke bron komt een citaat?
• Embedded question: ... uit welke bron een citaat komt.

This contrast is very useful for understanding Dutch word order.