Voordat de trein vertrekt, eet ik het broodje kaas op.

Voordat means before in the sense of before something happens.

It is a subordinating conjunction, which means it introduces a subordinate clause:

• Voordat de trein vertrekt = before the train leaves

Words like voordat, omdat, terwijl, and als often trigger a different word order in Dutch.

Because voordat introduces a subordinate clause, and in Dutch subordinate clauses the conjugated verb normally goes to the end.

So:

• voordat de trein vertrekt is correct

Here the clause only has one verb, vertrekt, so that verb comes after the subject de trein.

Compare:

• Main clause: De trein vertrekt
• Subordinate clause: voordat de trein vertrekt

In this example they look similar in English, but the Dutch rule is still important.

This is because Dutch main clauses follow the V2 rule: the finite verb must be in the second position.

The whole clause Voordat de trein vertrekt takes the first position in the sentence. So the verb of the main clause, eet, must come next, and the subject ik comes after it.

So:

• Voordat de trein vertrekt, eet ik het broodje kaas op.

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

• Ik eet het broodje kaas op voordat de trein vertrekt.

Both are correct, but the word order changes because of what comes first.

Because the verb is actually opeten, which is a separable verb.

In a main clause, separable verbs split:

• ik eet ... op

So:

• opeten → eet ... op

This is very common in Dutch:

• aandoen → ik doe mijn jas aan
• meenemen → ik neem het mee
• opeten → ik eet het op

Eten just means to eat.

Opeten means to eat up, to finish eating, or to consume completely.

So in this sentence, op adds the idea that the sandwich will be finished, not just eaten in general.

Compare:

• Ik eet een broodje. = I eat a sandwich.
• Ik eet een broodje op. = I eat up a sandwich / I finish the sandwich.

Because broodje is a diminutive, and all Dutch diminutives take the article het.

Broodje comes from brood plus the diminutive ending -je.

So:

• het broodje
• het huisje
• het meisje
• het boekje

This is a very useful rule: if a noun ends in a diminutive ending such as -je, -tje, -pje, -etje, it takes het.

Broodje kaas is a very common Dutch food expression.

• broodje = roll / sandwich
• kaas = cheese

Together, een broodje kaas means a cheese sandwich or a bread roll with cheese, depending on context.

A useful detail: kaasbroodje is a different word and often means a cheese pastry rather than a simple sandwich. So broodje kaas is the normal way to say cheese sandwich.

Dutch often uses the present tense for future events, especially when the future is clear from context.

In this sentence, the timing is already obvious because of voordat:

• Voordat de trein vertrekt clearly refers to a later event
• eet ik het broodje kaas op also refers to what will happen before that event

This is very natural Dutch. English does something similar in some cases:

• Before the train leaves, I eat the sandwich is less natural in English, but
• The train leaves at six uses present tense for the future too.

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

So:

• Voordat de trein vertrekt, eet ik het broodje kaas op.

If you reverse the order, you would normally still use a comma:

• Ik eet het broodje kaas op, voordat de trein vertrekt.

In modern usage, many writers prefer no comma before a short final subordinate clause in some contexts, but when learning Dutch, using the comma consistently with subordinate clauses is a safe habit.

Yes, absolutely.

That version is also correct:

• Ik eet het broodje kaas op voordat de trein vertrekt.

The main difference is focus:

• Voordat de trein vertrekt, eet ik het broodje kaas op. puts emphasis on the time frame first.
• Ik eet het broodje kaas op voordat de trein vertrekt. starts with the action itself.

This is a very common alternation in Dutch.