Toen ik thuiskwam, merkte ik dat het slot van de voordeur kapot was.

Here, toen means when.

Dutch toen is often used for a specific moment or event in the past:

• Toen ik thuiskwam... = When I got home...

It can also mean then in other contexts, but in this sentence it is a conjunction introducing a time clause.

A useful comparison:

• toen = when for a past event
• als = when / if for repeated events, general truths, or future situations

So here toen is correct because the sentence talks about one specific past moment.

Because thuiskomen is a separable verb.

Its basic form is:

• thuiskomen = to come home / to arrive home

In Dutch, separable verbs behave like this:

• In a main clause, they often split:
  • Ik kwam thuis.
• In a subordinate clause, they usually come back together:
  • toen ik thuiskwam

So:

• ik kwam thuis
• toen ik thuiskwam

That is why thuiskwam is one word here.

Because Toen ik thuiskwam is a subordinate clause.

In Dutch subordinate clauses, the conjugated verb normally goes to the end. That is one of the most important word-order differences from English.

So:

• English: when I came home
• Dutch: toen ik thuiskwam

The same thing happens later in the sentence:

• dat het slot van de voordeur kapot was

Again, the verb was goes to the end because that is also a subordinate clause.

Because Dutch main clauses follow the V2 rule: the conjugated verb must be in the second position.

The first position in this sentence is already taken by the whole clause:

• Toen ik thuiskwam

After that, the main-clause verb must come next:

• merkte

Then comes the subject:

• ik

So the structure is:

• Toen ik thuiskwam, merkte ik...

If you started directly with the subject, then you would get:

• Ik merkte dat het slot van de voordeur kapot was.

Both are grammatical, but once the sentence begins with Toen ik thuiskwam, the verb has to come before the subject in the main clause.

Because the sentence begins with a subordinate clause, and Dutch normally separates that clause from the main clause with a comma.

So:

• Toen ik thuiskwam, merkte ik...

This comma helps show where the time clause ends and the main clause begins.

It is very standard Dutch punctuation.

Because merken often introduces something that you notice or realize, and Dutch commonly expresses that with a dat-clause.

So:

• ik merkte dat... = I noticed that...

This is very similar to English:

• I noticed that the lock was broken

After dat, Dutch again uses subordinate-clause word order, so the verb goes to the end:

• dat het slot van de voordeur kapot was

Again, because this is a subordinate clause introduced by dat.

Dutch subordinate clauses usually put the finite verb at the end:

• dat ... was

The part kapot was means was broken.
Dutch often uses:

• subject + complement + verb

So:

• het slot ... kapot was

This may feel backwards to an English speaker, but it is normal Dutch subordinate-clause order.

Kapot means broken, damaged, or not working.

In this sentence, it most naturally means that the lock was not functioning properly. It does not have to mean the lock was physically smashed into pieces. It can simply mean it was defective or unusable.

Dutch uses kapot very often in everyday language:

• Mijn fiets is kapot.
• De wasmachine is kapot.
• Het slot is kapot.

So it is a very common, natural word here.

Because Dutch nouns have grammatical gender, and that affects the article.

• het slot: slot is a het-word
• de voordeur: voordeur is a de-word

Unfortunately, this is something you usually have to learn noun by noun.

In this phrase:

• het slot van de voordeur means the lock of the front door

So the articles are simply the normal ones for those nouns.

Yes, you could.

Dutch often makes compound nouns, so:

• het voordeurslot = the front-door lock

That is perfectly natural Dutch.

The version in your sentence,

• het slot van de voordeur is also very natural and may sound a little more explicit or descriptive.

So both are possible:

• het slot van de voordeur
• het voordeurslot

Because the sentence tells a past event and the situation connected to it.

• thuiskwam = the moment of arriving home
• merkte = the act of noticing
• was = the state the lock was in at that time

Dutch often uses the simple past in storytelling and narration, especially in sentences like this.

So the tense sequence is very natural:

1. I got home.
2. I noticed something.
3. That thing was broken.

In standard spelling for this sentence, thuiskwam is the expected form because it comes from the separable verb thuiskomen.

Compare:

• Ik kwam thuis.
  Main clause: split
• Toen ik thuiskwam.
  Subordinate clause: joined

So for learners, the safest rule is:

• main clause: kwam thuis
• subordinate clause / infinitive / participle: thuiskomen, thuiskwam, thuisgekomen

That is why thuiskwam is the form you should learn here.