A diák hibát talál a mondatban, ezért megkérdezi a tanárt.

Because Hungarian often leaves an indefinite singular noun without any article, especially as a direct object.

So hibát talál is a natural way to say finds a mistake. It does not need egy.

• hibát talál = finds a mistake / finds mistakes
• egy hibát talál = finds one mistake, or puts a bit more emphasis on the fact that it is a single mistake

So the bare noun hibát is completely normal here.

The -t is the accusative ending, which marks the direct object of the verb.

In this sentence:

• hibát = a mistake, as the object of talál
• a tanárt = the teacher, as the object of megkérdezi

This is one of the most important case endings in Hungarian.

A small detail: the basic forms are:

• hiba → hibát
• tanár → tanárt

The final -t is the key part you should notice.

Here a is the Hungarian definite article, equivalent to English the.

So:

• a diák = the student
• a mondatban = in the sentence
• a tanárt = the teacher

Hungarian has two forms of the definite article:

• a before a consonant sound
• az before a vowel sound

Since diák, mondat, and tanár all begin with consonant sounds, the correct form is a.

mondatban is mondat plus the case ending -ban, which means in.

• mondat = sentence
• mondatban = in the sentence

So instead of using a separate word like English in, Hungarian usually adds a case ending directly to the noun.

Also notice that the article stays separate:

• a mondatban = in the sentence

The ending goes on the noun, not on the article.

This is because Hungarian verbs have two main conjugation patterns:

• indefinite conjugation
• definite conjugation

They depend mainly on whether the object is indefinite or definite.

In the sentence:

• hibát talál: hibát is indefinite, so the verb is talál
• a tanárt megkérdezi / megkérdezi a tanárt: a tanárt is definite, so the verb is megkérdezi

So:

• talál = he/she finds something indefinite
• megkérdezi = he/she asks that specific person

This definite vs. indefinite verb agreement is a very characteristic feature of Hungarian.

meg- is a verbal prefix. In many verbs, it gives the action a more complete, bounded, or single-event feeling.

With kérdez:

• kérdez = asks / is asking / questions
• megkérdez = asks someone, asks as a complete action

In many contexts, English translates both simply as ask, but Hungarian often prefers megkérdez when someone asks a specific question or addresses someone directly.

So in this sentence, megkérdezi a tanárt is the natural way to say asks the teacher.

In Hungarian, the verb megkérdez takes the person asked as a direct object.

So:

• megkérdez valakit = ask someone

That is why we get:

• a tanárt with the accusative ending -t

This is simply the normal verb pattern in Hungarian.
You do not use a dative-like structure here.

Hungarian usually omits subject pronouns when they are already clear from the context.

So instead of saying something equivalent to therefore he/she asks the teacher, Hungarian simply says:

• ezért megkérdezi a tanárt

The subject is understood to be the same as before: a diák.

This is very common in Hungarian.

Yes, Hungarian word order is flexible, and it often reflects topic and focus, not just grammar.

Here the sentence has a fairly neutral flow:

• A diák = topic, what we are talking about
• hibát talál = finds a mistake
• a mondatban = in the sentence

So the sentence feels like a straightforward statement about the student.

Other word orders are possible, but they would change the emphasis. For example, moving a mondatban earlier could put more attention on where the mistake is found.

So this order is natural, but it is not the only grammatical one.

ezért means therefore, for this reason, or so.

It connects the two parts of the sentence:

• the student finds a mistake
• as a result, the student asks the teacher

So the logic is:

X happens, ezért Y happens.

The comma is used because there are two clauses:

• A diák hibát talál a mondatban
• ezért megkérdezi a tanárt

In English, you would often also separate this kind of structure with punctuation.