Learn Hardcore German: Die Küche muss heute gründlich gereinigt werden. - The kitchen must be cleaned thoroughly today.

Active: Jemand muss die Küche heute gründlich reinigen.
“Someone must clean the kitchen thoroughly today.”
Passive: Die Küche muss heute gründlich gereinigt werden.
“The kitchen must be thoroughly cleaned today.”

In German passive with a modal verb (müssen, können, etc.), you use:

finite modal verb: muss (2nd position)
past participle of the main verb: gereinigt
infinitive werden at the end

Pattern: Subjekt – Modalverb – … – Partizip II – werden
→ Die Küche muss … gereinigt werden.

So reinigen is the active infinitive; gereinigt werden is the passive infinitive (“to be cleaned”).

Is this sentence in the passive voice? How can I recognize that?

Yes, it is a passive sentence.

You can recognize the German passive by:

Form:
- auxiliary werden
- plus a past participle (here: gereinigt)
Meaning / role of the subject:
- The subject Die Küche is not doing the action; it receives the action.
- In active voice, the subject would be the person cleaning, e.g. Ich reinige die Küche.

So the structure wird / werden + Partizip II (here with a modal: muss … gereinigt werden) is your big passive clue.

Who is actually doing the cleaning? Why is there no “by someone” in the German sentence?

The sentence does not say who is doing the cleaning. This is very normal in German passive sentences.

German often omits the agent (“by someone”) when it is:
- obvious from context, or
- unimportant, or
- a general “people/they/you” meaning.

If you do want to say who is doing it, you typically use von + Dativ:

Die Küche muss heute gründlich von mir gereinigt werden.
“The kitchen must be thoroughly cleaned by me today.”

But in everyday language, Germans frequently leave the agent out, just like in English:
“The kitchen must be cleaned today” (no “by X” stated).

What is the difference between gereinigt, geputzt, and sauber gemacht? Can I replace them?

All can refer to cleaning, but there are nuances:

reinigen / gereinigt
- Slightly more formal / technical.
- Common for professional or thorough cleaning: machines, rooms, surfaces.
- In your sentence, gereinigt sounds a bit neutral to formal, perfectly fine.
putzen / geputzt
- Very everyday and common.
- Typical for housework: die Küche putzen, das Bad putzen, Fenster putzen.
sauber machen / sauber gemacht
- Very colloquial, literally “to make clean”.
- Often used in spoken language: die Küche sauber machen.

You could say:

Die Küche muss heute gründlich geputzt werden.
Die Küche muss heute gründlich sauber gemacht werden.

All are acceptable, but gereinigt is a bit more neutral/standard and fits well with gründlich.

Why is it Die Küche and not Der Küche here? Which case is that?

Die Küche is in the nominative case, because it is the subject of the sentence.

The passive subject is the thing being acted on, so Die Küche (the kitchen) is the subject that “must be cleaned”.

For a typical feminine noun like Küche, the definite article is:

Nominative singular: die Küche
Accusative singular: die Küche
Dative singular: der Küche
Genitive singular: der Küche

Here we need the subject form → Nominative, so die Küche, not der Küche.

What exactly does muss express here? Could I use soll instead?

muss (from müssen) expresses necessity / obligation:

Die Küche muss heute gründlich gereinigt werden.
→ It is necessary; it really has to happen.

Alternatives:

soll (from sollen)
- More like “is supposed to / should”.
- Slightly weaker, may come from an external rule or plan.
- Die Küche soll heute gründlich gereinigt werden.
  → The plan/instruction is that it be cleaned.
sollte
- “should” in a softer, more advisory sense.
- Die Küche sollte heute gründlich gereinigt werden.
  → It would be a good idea for it to be cleaned today.

So muss is the strongest in terms of obligation.

Can I change the word order, for example to Heute muss die Küche gründlich gereinigt werden?

Yes, that is perfectly correct, and very natural.

Both are fine:

Die Küche muss heute gründlich gereinigt werden. (neutral focus)
Heute muss die Küche gründlich gereinigt werden. (emphasis on today)

In main clauses, German normally keeps:

Finite verb in 2nd position: muss
Other elements (subject, adverbs, objects) can be placed in the 1st position to be highlighted.

So you can also say:

Gründlich muss die Küche heute gereinigt werden. (emphasis on “thoroughly” – stylistically marked)

But the passive verb part gereinigt werden must stay together at the end.

Where can heute and gründlich appear in the sentence? Is there a rule for their order?

A common guideline for adverb order in German is:

Time – Manner – Place

In your sentence:

heute = Time
gründlich = Manner (how it is cleaned)

So the order heute gründlich fits that pattern:

Die Küche muss heute gründlich gereinigt werden.

Other acceptable placements (with slight changes in emphasis) are:

Heute muss die Küche gründlich gereinigt werden. (focus on today)
Die Küche muss gründlich heute gereinigt werden. (possible, but sounds less natural)

The safest and most neutral in a main clause is Time → Manner: heute gründlich.

Why is gereinigt at the end, and why does werden also go to the end?

German likes to put non-finite verb forms (infinitives, participles) at the end of the clause.

In a main clause with a modal verb:

The finite modal verb goes in 2nd position: muss
The rest of the verb phrase goes to the end.

Here the rest is the passive construction gereinigt werden:

Die Küche – subject
muss – finite verb in 2nd position
heute gründlich – adverbials
gereinigt werden – non-finite verb cluster at the end

So the basic pattern is:

Subjekt – (other elements) – Modalverb – (other elements) – Partizip II – werden

→ Die Küche muss heute gründlich gereinigt werden.

How would this sentence look in a subordinate clause, for example after weil?

In a subordinate clause introduced by weil, the finite verb goes to the very end of the clause.

So:

Main clause:
Die Küche muss heute gründlich gereinigt werden.
Subordinate clause:
…, weil die Küche heute gründlich gereinigt werden muss.

Order in the subordinate clause end cluster:

gereinigt (participle)
werden (infinitive)
muss (finite modal, last position in the clause)

Pattern: …, weil [Subjekt … Partizip II – werden – Modalverb].

Can I use sein instead of werden, like Die Küche muss heute gründlich gereinigt sein?

You can say gereinigt sein, but it changes the meaning.

gereinigt werden = process passive
- Focus on the action taking place:
  “must be (get) cleaned”
gereinigt sein = result state
- Focus on the resulting condition after cleaning:
  “must be in a cleaned state”

Compare:

Die Küche muss heute gründlich gereinigt werden.
→ The act of cleaning must happen today.
Die Küche muss heute gründlich gereinigt sein.
→ By today, the kitchen must be in a thoroughly cleaned state (maybe the deadline is sometime today).

So werden = action; sein = result/state.

Why doesn’t gereinigt change its form to agree with die Küche (feminine), like an adjective would?

Here gereinigt is not acting as an adjective, but as part of a verb phrase (a past participle in the passive construction).

As a verbal participle, it does not agree in gender, number, or case with the subject. It stays in its fixed participle form: gereinigt.

You would only see agreement if gereinigt were used as an adjective directly before a noun:

die gereinigte Küche = “the cleaned kitchen”
Here gereinigte agrees with die Küche (feminine nominative singular).

In the sentence Die Küche muss heute gründlich gereinigt werden, gereinigt belongs to the verb, not to the noun.

How do I turn this into a yes/no question, like “Does the kitchen need to be thoroughly cleaned today?”

In a yes/no question, German puts the finite verb first.

So from:

Die Küche muss heute gründlich gereinigt werden.

you get:

Muss die Küche heute gründlich gereinigt werden?
“Does the kitchen need to be thoroughly cleaned today?”

Structure:

Muss – finite verb first
die Küche – subject
heute gründlich – adverbials
gereinigt werden – non-finite verb cluster at the end

How do I say this in the past: “The kitchen had to be thoroughly cleaned”?

You can use Präteritum of müssen with the passive:

Die Küche musste gründlich gereinigt werden.
“The kitchen had to be thoroughly cleaned.”

If you need to be explicit about time:

Die Küche musste gestern gründlich gereinigt werden.
“The kitchen had to be thoroughly cleaned yesterday.”

Using a perfect form (hat … müssen gereinigt werden) is possible but usually avoided in everyday speech because it becomes clumsy; musste … gereinigt werden is the normal, natural version.