Ki kell cserélni a hibás féket.

Here ki is a verbal prefix. On its own, ki often means out, but in kicserélni it helps form the verb to replace / to exchange.

So:

• cserélni = to change / to exchange
• kicserélni = to replace, literally something like to exchange out

In this sentence, ki is not a separate word with an independent meaning like out in English. It belongs to the verb.

This is a very common Hungarian pattern.

The full verb is kicserélni, but when a verb with a prefix appears with kell + infinitive, the prefix often moves in front of kell:

• Ki kell cserélni
  instead of
• Kicserélni kell

So the sentence is built like this:

• ki = verbal prefix
• kell = must / need to
• cserélni = to replace

This split is normal and very frequent in Hungarian.

Kell means must, have to, or need to.

In this sentence, it creates an impersonal must be replaced / needs to be replaced type of meaning.

So ki kell cserélni literally means something like:

• must replace
• needs to be replaced
• has to be replaced

The exact English translation depends on context.

Because after kell, Hungarian usually uses the infinitive.

So this pattern is very common:

• kell + infinitive

Examples:

• menni kell = one must go / it is necessary to go
• meg kell csinálni = it must be done
• ki kell cserélni = it must be replaced

So cserélni is in the infinitive because kell requires it in this structure.

Exactly — Hungarian often leaves that unspecified.

Ki kell cserélni a hibás féket is an impersonal statement. It means something like:

• The faulty brake must be replaced
• Someone has to replace the faulty brake

The sentence focuses on the necessity of the action, not on the person doing it.

If you wanted to say who must do it, Hungarian could express that differently, for example with a personal ending or an explicit subject.

Because féket is in the accusative case, which marks the direct object.

• fék = brake
• féket = brake as the thing being acted on

Here, the brake is what must be replaced, so Hungarian marks it with -t.

This is one of the most important Hungarian patterns:

• direct object → usually gets -t

A is the definite article, meaning the.

So:

• a hibás féket = the faulty brake
• egy hibás féket = a faulty brake

Using a suggests a specific brake is being talked about — not just any faulty brake, but the one relevant in the situation.

Because in Hungarian, an adjective used before a noun usually does not take the case ending. The noun carries the case ending, not the adjective.

So:

• hibás fék = faulty brake
• hibás féket = faulty brake in the accusative

Only the noun changes:

• fék → féket

The adjective hibás stays the same.

Hibás means faulty, defective, or bad in the sense of not working properly.

It comes from hiba = fault / error / defect.

So a hibás fék is:

• the faulty brake
• the defective brake

Yes, féket is grammatically singular in Hungarian.

• fék = brake
• fékek = brakes

However, English sometimes uses brake or brakes differently depending on context. So even though the Hungarian is singular, a natural English translation might vary depending on the situation.

But strictly grammatically, féket is singular accusative.

Yes. That is also correct.

Hungarian word order is flexible, and moving elements changes the focus or emphasis.

Compare:

• Ki kell cserélni a hibás féket.
  Neutral: The faulty brake needs to be replaced.

• A hibás féket ki kell cserélni.
  More emphasis on the faulty brake as the thing that must be replaced.

Both are grammatical. The difference is mostly about information structure and emphasis.

This is a useful vocabulary point.

• cserélni = to change / exchange
• kicserélni = to replace, often with a sense of removing the old one and putting in another one

So in this sentence, ki makes the meaning more specific: not just change, but replace.

For example:

• Olajat kell cserélni. = The oil needs changing.
• Ki kell cserélni a féket. = The brake has to be replaced.

It is neutral and standard Hungarian.

It sounds natural in everyday speech, technical contexts, repair situations, and written instructions.

So it could fit in places like:

• a mechanic's comment
• a repair report
• a maintenance instruction

It is neither especially formal nor especially slangy.

Then Hungarian would usually show who has to do it.

A common way is:

• Ki kell cserélnünk a hibás féket.

Here:

• cserélnünk = for us to replace / we must replace

So:

• Ki kell cserélni a hibás féket. = The faulty brake must be replaced. / One must replace the faulty brake.
• Ki kell cserélnünk a hibás féket. = We have to replace the faulty brake.

That is a very useful contrast to learn.