Learn Hardcore Dutch: Toen ik thuiskwam, zag ik dat de keuken vol rommel lag. - When I came home, I saw that the kitchen was full of mess.

toen is a subordinating conjunction used for a single, completed event in the past (“when I arrived home”). It introduces a time clause. Key points:

Use toen for specific past moments.
It sends the finite verb to the end of its clause (here: thuiskwam).
Contrast with wanneer, which appears in questions or for repeated/habitual events.

How is thuiskwam formed? Why isn’t it written as ik kwam thuis?

thuiskwam is the simple past of the separable verb thuiskomen (to come home) in a subordinate clause. Notice:

Prefix thuis
- past stem kwam = thuiskwam (one word).
In a main clause you separate them: ik kwam thuis.
In subordinate clauses (after toen) you write the past tense of a separable verb together.

Why is the main clause zag ik inverted instead of saying ik zag?

Because the sentence began with a subordinate time clause (Toen ik thuiskwam,), the following main clause undergoes subject–verb inversion. So you get:
verb first (zag), then subject (ik).

What role does dat play in zag ik dat de keuken vol rommel lag?

dat is a conjunction meaning that, introducing an indirect statement (content clause). It links the verb zag (saw) to what was seen: “that the kitchen was full of mess.”

Why does the verb lag appear at the end in dat de keuken vol rommel lag?

Because dat introduces a subordinate clause, Dutch word order requires the finite verb at the end. Structure here:
subject (de keuken) → predicate complement (vol rommel) → verb (lag).

What does vol rommel mean? How does vol function here?

vol means full of. When followed by a noun phrase, it indicates abundance of that noun.
Example: vol rommel = full of mess/junk.

Could I have used an adjective like rommelig instead of vol rommel?

Yes.

dat de keuken rommelig was = that the kitchen was messy.
vol rommel lag emphasizes physical clutter lying around, whereas rommelig gives a general quality.

Why is rommel an uncountable noun here? Could I say een rommel?

As mess or junk, rommel is normally uncountable when referring to general clutter.

rommel = (uncountable) clutter.
een rommel would sound like “a worthless object” rather than “a mess.”

Why did we use lag (past of liggen) and not stond (past of staan)?

liggen describes things lying flat or scattered, fitting for stuff strewn around. staan implies upright position. Here, the mess is lying in the kitchen, so liggen (lag) is appropriate.

Could I re-order the clauses and still be correct? For example: Ik zag dat de keuken vol rommel lag toen ik thuiskwam.

Yes. That order places the main clause first and the time clause at the end. You keep:

lag at the end of the dat-clause.
thuiskwam at the end of the toen-clause.
The emphasis shifts slightly to focus on seeing first rather than when.