Der Hausmeister hat die leere Batterie im Rauchmelder sofort ausgetauscht.

The caretaker replaced the empty battery in the smoke detector immediately.

Breakdown of Der Hausmeister hat die leere Batterie im Rauchmelder sofort ausgetauscht.

• haben: to have
• im: in the; (masculine or neuter, dative)
• sofort: immediately
• leer: empty
• die Batterie: the battery
• die: the; (feminine, accusative)
• der Hausmeister: the caretaker
• der Rauchmelder: the smoke detector
• austauschen: to replace
• der: the; (feminine, genitive/dative)

Questions & Answers about Der Hausmeister hat die leere Batterie im Rauchmelder sofort ausgetauscht.

Why is it Der Hausmeister and not Die Hausmeister or Den Hausmeister?

Der Hausmeister is masculine singular in the nominative case, because it’s the subject (the person doing the action).

• die Hausmeister would usually be plural (the caretakers).
• den Hausmeister would be accusative (used if the caretaker were the direct object, e.g., Ich sehe den Hausmeister.).

What tense is hat … ausgetauscht?

It’s the Perfekt (present perfect), commonly used in spoken German to talk about past actions.
Structure: haben/sein (conjugated) + Partizip II (past participle)
Here: hat (from haben) + ausgetauscht (Partizip II).

Why does the verb come in two parts: hat … ausgetauscht?

That’s the normal Perfekt word order:

• The conjugated auxiliary (hat) is in position 2 in a main clause.
• The past participle (ausgetauscht) goes to the end of the clause.

Is austauschen a separable verb? Does that matter here?

Yes, austauschen is separable (aus- + tauschen).
In the present tense, you’d see the separation: Der Hausmeister tauscht … aus.
In the Perfekt, it stays together as one participle: ausgetauscht (with ge in the middle).

Why is it die leere Batterie and not der leere Batterie or die leeren Batterie?

Because Batterie is a feminine noun (die Batterie) and here it’s in the accusative singular as the direct object.
Adjective ending depends on gender/case and the article:

• die + Batterie (fem. acc. sg.) → die leer-e Batterie
  Not:
• der (wrong gender/article)
• leeren (that would typically be plural or a different case pattern).

How do I know die leere Batterie is accusative?

Because it answers What did he exchange?
German verbs like austauschen usually take a direct object in the accusative.
In this sentence, die Batterie is the thing being replaced.

What’s the function of im Rauchmelder? Why im?

im is a contraction of in dem.

• in can take dative for location (where?)
• dem is dative masculine/neuter singular
  So im Rauchmelder = in the smoke detector (location: where the battery is).

Why is it Rauchmelder and not something like Rauchdetektor?

Rauchmelder is the common everyday German word for smoke alarm/smoke detector. It’s a compound noun:

• Rauch = smoke
• Melder = device that reports/signals
  Rauchdetektor exists but Rauchmelder is far more typical in normal usage.

Where does sofort usually go in a sentence, and could it move?

sofort is an adverb of time/urgency. Its position is flexible, but meaning and emphasis can shift slightly. Common options:

• Der Hausmeister hat … sofort ausgetauscht. (neutral, very common)
• Der Hausmeister hat sofort die leere Batterie … ausgetauscht. (emphasizes immediacy early)
• Sofort hat der Hausmeister … ausgetauscht. (strong emphasis on immediately; stylistically marked)

Why is the participle ausgetauscht at the very end—does anything ever come after it?

In a normal main clause, the Partizip II comes at the end. Some things can come after it, but they’re special cases (e.g., afterthoughts, heavy additions, certain comparative phrases). The basic rule you should learn is: non-conjugated verb forms go to the end.

Could this sentence also be in Präteritum? What would it look like?

Yes. Präteritum is more common in writing (and with some very common verbs in speech). In Präteritum:
Der Hausmeister tauschte die leere Batterie im Rauchmelder sofort aus.
You can see the separable verb clearly: tauschte … aus.

Does Batterie mean a regular battery, and is Akku different?

Yes, die Batterie is typically a disposable/standard battery (like AA/AAA or a 9V battery, common in smoke alarms).
der Akku usually means a rechargeable battery (battery pack). In many smoke alarms, Batterie is the expected word.