A hibákat kötelező kijavíttatni.

Hibákat is the plural accusative form of hiba meaning mistake or error.

• hiba = mistake
• hibák = mistakes
• hibákat = mistakes as the direct object

The -t marks the accusative in Hungarian, because these mistakes are the things being corrected.

Because Hungarian uses the same definite article a/az for both singular and plural nouns.

So:

• a hiba = the mistake
• a hibák = the mistakes
• a hibákat = the mistakes (as object)

There is no separate word for plural the in Hungarian.

Kötelező means obligatory, mandatory, or required.

In this sentence it works as an impersonal predicate:
kötelező + infinitive = it is obligatory to ...

So the structure is basically:

• A hibákat kötelező kijavíttatni.
• It is obligatory to have the mistakes corrected.

Hungarian does not need a dummy subject like English it.

It contains several pieces:

• ki- = a verbal prefix
• javít = correct, fix
• -tat/-tet = causative suffix, often meaning have someone do something
• -ni = infinitive ending, to ...

So:

• kijavítani = to correct
• kijavíttatni = to have something corrected

The whole verb means not that you correct the mistakes yourself, but that you arrange for someone else to correct them.

This is a very important difference.

• kijavítani = to correct
• kijavíttatni = to have something corrected

So:

• Ki kell javítani a hibákat. = The mistakes must be corrected.
• Ki kell javíttatni a hibákat. = The mistakes must be had corrected / someone must get them corrected.

The second one implies that another person does the correcting.

That is normal. The sentence is impersonal, so the responsible person is left unstated.

Depending on context, it could mean:

• a student
• an employee
• the person in charge
• people in general

Hungarian often leaves this kind of subject unexpressed when it is obvious from the situation or when the statement is meant generally.

If you wanted to name the person, you could say something like:

• A diáknak kötelező kijavíttatni a hibákat.
  The student is required to have the mistakes corrected.

Hungarian word order is much more about topic and emphasis than fixed English-style sentence order.

Starting with A hibákat puts the mistakes in the topic position. It can sound like:

• As for the mistakes, it is obligatory to have them corrected.

A more neutral version could be:

• Kötelező kijavíttatni a hibákat.

Both are grammatical, but the original sentence gives special prominence to the mistakes.

Yes, absolutely.

That version is very natural and often more common in everyday Hungarian.

The difference is mainly in style:

• kötelező kijavíttatni = more formal, rule-like, official
• ki kell javíttatni = more ordinary, practical must have corrected

So the original sentence sounds a bit more like a regulation or requirement.

In many verbs, ki- originally has the basic meaning out, but in prefixed verbs it often gives a more specific, established meaning.

With javít, the form kijavít is the normal verb for correct, especially in the sense of correcting mistakes in writing, work, exercises, and so on.

So here you should understand kijavíttatni as a whole lexical unit meaning to have corrected, not as a literal word-for-word correct out.

Because the base verb already ends in -t:

• javít

Then the causative suffix is added:

• -tat/-tet

So the result is:

• javít + tat → javíttat

That is why you see the doubled tt. It reflects the way the parts join together. This is a normal spelling pattern in Hungarian.