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

Because the sentence starts with another element: Toen ik thuiskwam.

In a Dutch main clause, the finite verb normally comes in second position. If the sentence begins with the subject, you get:

• Ik merkte ...

But here the sentence begins with a time clause:

• Toen ik thuiskwam, ...

So the finite verb of the main clause must come next:

• Toen ik thuiskwam, merkte ik ...

This is a very common Dutch pattern. English does not usually do this inversion, so it often feels strange at first.

Here toen means when, referring to a specific moment in the past.

• Toen ik thuiskwam = when I got home

Dutch often uses:

• toen for a specific event in the past
• als for repeated events, general truths, or conditions

So:

• Toen ik thuiskwam, ... = one particular time in the past
• Als ik thuiskom, ... = when/whenever I come home

That is why toen is the natural choice here.

Because the verb is thuiskomen = to come home.

This is treated as a separable verb:

• infinitive: thuiskomen
• main clause: Ik kwam thuis
• subordinate clause: ... dat ik thuiskwam

In a subordinate clause, the separable parts usually come back together at the end:

• Toen ik thuiskwam

So English learners often notice two forms:

• Ik kwam thuis
• Toen ik thuiskwam

Both come from thuiskomen.

Because Toen ik thuiskwam is a subordinate clause.

In Dutch subordinate clauses, the finite verb typically goes toward the end. So:

• ik kwam thuis = main clause
• toen ik thuiskwam = subordinate clause

You can think of it like this:

• main clause: normal statement word order
• subordinate clause: verb moved to the end area

That is one of the most important word-order differences between Dutch and English.

Dat here means that and introduces a clause explaining what I noticed.

So:

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

The clause after dat is also a subordinate clause, so the verb goes to the end:

• dat het slot van de deur kapot was

Without dat, the sentence would not sound complete in standard Dutch for this meaning.

Because the part after dat is a subordinate clause:

• dat het slot van de deur kapot was

In subordinate clauses, the finite verb usually comes at the end. That is why was comes after kapot.

Compare:

• main clause: Het slot was kapot.
• subordinate clause: ... dat het slot kapot was.

This is a very common Dutch pattern:

• Ik zag dat hij moe was.
• Ze zei dat het laat was.

Because slot is a het-word in Dutch.

So the correct definite article is:

• het slot = the lock

This is just something that has to be learned with the noun. Dutch nouns can take either:

• de
• het

and slot belongs to the het group.

Slot means lock.

In this sentence, it refers to the lock of the door:

• het slot van de deur = the lock of the door

Be careful not to confuse it with English words like slot meaning a narrow opening or time slot. In Dutch, slot very commonly means lock.

Dutch often expresses this kind of relationship with van:

• het slot van de deur = the lock of the door

English usually prefers:

• the door lock
• the lock of the door (less common in everyday English)

Dutch can sometimes also use a compound noun:

• het deurslot

Both are possible, but het slot van de deur is very clear and natural.

Here kapot means broken.

Grammatically, it is used as a predicative adjective, meaning it comes after a linking verb like zijn:

• Het slot was kapot. = The lock was broken.

If you put it before the noun, it changes form:

• het kapotte slot = the broken lock

So:

• kapot after was
• kapotte before slot

That difference is very common with Dutch adjectives.

Because this sentence tells about a past situation as part of a narrative.

Dutch often uses the simple past for storytelling and for describing what happened:

• Toen ik thuiskwam, merkte ik dat ... was.

In spoken Dutch, especially in conversation, the perfect tense is also very common, and you might hear something like:

• Toen ik thuis kwam, heb ik gemerkt dat het slot van de deur kapot was.

But in a natural narrative sentence, the simple past is very normal and often preferred.

You may sometimes see thuis kwam written separately, especially in less formal contexts, but the standard form with the separable verb thuiskomen is thuiskwam in this kind of clause.

The important pattern to remember is:

• infinitive: thuiskomen
• main clause: Ik kwam thuis
• subordinate clause: ... toen ik thuiskwam

So for learners, thuiskwam is the form to aim for here.

It has three layers:

1. Toen ik thuiskwam
   a time subordinate clause

2. merkte ik
   the main clause

3. dat het slot van de deur kapot was
   another subordinate clause, depending on merkte

So the structure is roughly:

• [When I got home], [I noticed] [that the door lock was broken].

This sentence is a very useful example because it shows:

• subordinate clause word order
• inversion in the main clause
• another subordinate clause after dat