Learn Hardcore German: Ich ersetze das alte Kabel, weil es kaputt ist. - I am replacing the old cable because it is broken.

In the sentence, das alte Kabel is the direct object (what is being replaced), so it is in the accusative case.
For neuter nouns, the nominative and accusative articles are the same:

Nominative: das Kabel (the cable is here)
Accusative: das Kabel (I replace the cable)

So you get das (not den) and then the adjective with its ending: das alte Kabel.

Why is the adjective alte and not alt or altes?

The form of the adjective depends on:

The article in front of the noun
The gender of the noun
The case (nominative, accusative, etc.)

Here:

Article: das (definite article)
Gender: neuter (Kabel)
Case: accusative (direct object)

After a definite article, the adjective takes the weak ending.
For neuter singular in the accusative, that ending is -e.

So:

das alte Kabel = the old cable (accusative, neuter, with definite article)

If there were no article, you would say:

altes Kabel (here the adjective takes -es, a strong ending)

Why is there a comma before weil?

In German, subordinate clauses (Nebensätze) introduced by conjunctions like weil, dass, wenn, obwohl etc. are normally separated from the main clause by a comma.

Main clause: Ich ersetze das alte Kabel
Subordinate clause: weil es kaputt ist

German orthography requires a comma between them:

Ich ersetze das alte Kabel, weil es kaputt ist.

Unlike English, where the comma before because is often optional, in German the comma before weil is mandatory.

Why is the verb ist at the end of the clause weil es kaputt ist?

Weil is a subordinating conjunction. In German, subordinate clauses have different word order:

In a main clause, the conjugated verb is in second position:
- Es ist kaputt. (It is broken.)
In a subordinate clause with weil, the conjugated verb goes to the end:
- weil es kaputt ist (because it is broken)

So:

Main clause: Es ist kaputt.
Subordinate clause: ..., weil es kaputt ist.

You cannot say: weil es ist kaputt – that is incorrect in standard German.

Could I also say ..., weil das Kabel kaputt ist instead of ..., weil es kaputt ist?

Yes, you can. Both are grammatically correct:

Ich ersetze das alte Kabel, weil es kaputt ist.
Ich ersetze das alte Kabel, weil das Kabel kaputt ist.

Using es avoids repeating das Kabel and sounds more natural, just like English prefers it instead of repeating the noun:

I replace the old cable because it is broken.
(Not usually: because the cable is broken.)

In German, es is the standard pronoun for neuter nouns like das Kabel.

Why do we use es for Kabel? Is there any other option for the pronoun?

Kabel is grammatically neuter, so its third-person singular pronoun is es:

das Kabel → es
der Tisch (masc.) → er
die Lampe (fem.) → sie

There isn’t another pronoun that would be correct here. You must use es when referring back to das Kabel in the singular.

Why is it ersetze and not something like wechsle, tausche or austausche?

All of these can involve changing something, but there are nuances:

ersetzen = to replace something with something else, to take its place
- Focus: the old thing is removed and a new one takes over its function.
- Ich ersetze das alte Kabel. → I remove the old cable and put a new one in its place.
wechseln = to change/switch (often used with clothes, tires, etc.)
- Ich wechsle das Kabel. is possible, but sounds more like “I’m changing the cable” without emphasizing that one replaces the other.
(aus)tauschen = to exchange, swap out, replace
- Ich tausche das alte Kabel aus. (separable verb)
- Similar meaning to ersetzen, common in technical contexts.

In your sentence, ersetzen clearly expresses that the old cable is being replaced by another one; it’s a very natural choice.

Is ersetzen a separable verb? Why is it written as one word?

Ersetzen is not a separable verb.

Prefix er- in German is generally inseparable.
That means the prefix never splits off and moves to the end of the clause.

So you always keep it together:

Ich ersetze das alte Kabel.
Ich habe das alte Kabel ersetzt.
Ich werde das alte Kabel ersetzen.

Compare this to a separable verb like austauschen:

Ich tausche das alte Kabel aus.
Ich habe das alte Kabel ausgetauscht.

Here aus- splits away in certain tenses, but er- never does.

Why is the verb ersetze in the present tense here? Could I use it for a future action?

German uses the present tense (Präsens) more broadly than English. It can cover:

Present: Ich ersetze das alte Kabel. → I am replacing the old cable (right now).
Near future (with a future meaning from context):
Ich ersetze das alte Kabel morgen. → I’ll replace the old cable tomorrow.

If you really want to stress the future, you can use Futur I:

Ich werde das alte Kabel ersetzen.

But in everyday speech, the present tense with a time expression like morgen, gleich, heute Nachmittag is usually enough.

Is kaputt informal? Are there more formal or technical alternatives?

Kaputt is very common in everyday speech and is slightly informal, but still perfectly acceptable in most situations.

Alternatives include:

defekt – more formal/technical
- Das Kabel ist defekt.
beschädigt – damaged
- Das Kabel ist beschädigt.
zerbrochen – broken into pieces (more physical breaking, usually hard objects)
- Works better with things like Glas, Tasse, Fenster.

For a cable, kaputt and defekt are both common.
Your sentence with kaputt sounds natural and normal.

Could I say Ich ersetze das Kabel, denn es ist kaputt instead of using weil? What’s the difference?

Yes, you can say:

Ich ersetze das alte Kabel, denn es ist kaputt.

Differences:

Word order
- With weil (subordinating conjunction), the verb goes to the end:
  - ..., weil es kaputt ist.
- With denn (coordinating conjunction), the word order stays like in a main clause:
  - ..., denn es ist kaputt.
Style/feeling
- weil = because; used everywhere, neutral, very common in spoken and written German.
- denn = also “because”, but a bit more written or formal, and slightly less common in casual speech.

Meaning-wise they are almost the same in this context. The main change is word order.

Could the sentence be just Das alte Kabel ist kaputt.? How does that differ from the original?

Yes:

Das alte Kabel ist kaputt.
= The old cable is broken.

This only states the fact that the cable is broken.

The original sentence:

Ich ersetze das alte Kabel, weil es kaputt ist.
= It adds a reason and an action: you are replacing it because it is broken.

So:

Das alte Kabel ist kaputt. → state of the cable
Ich ersetze das alte Kabel, weil es kaputt ist. → what you are doing and why