Toen het onweer begon, had het publiek de bioscoop al verlaten.

When the thunderstorm started, the audience had already left the cinema.

Breakdown of Toen het onweer begon, had het publiek de bioscoop al verlaten.

• hebben: to have
• al: already
• toen: when
• beginnen: to start
• verlaten: to leave
• de bioscoop: the cinema
• het publiek: the audience
• het onweer: the thunderstorm

Questions & Answers about Toen het onweer begon, had het publiek de bioscoop al verlaten.

What does toen mean in this sentence, and how is it different from wanneer or als?

Toen here is a subordinating conjunction meaning “when” for a specific moment in the past. You use toen to link two past events (e.g. “When the thunderstorm started…”).

• Wanneer is used in questions (“Wanneer begon het onweer?”) or in more formal written contexts for “when.”
• Als is used for general or repeated conditions (“Als het regent, blijf ik thuis” – “Whenever/if it rains, I stay home”), not for a single past occurrence.

Why is the verb begon placed at the end of the clause?

Because it’s a subordinate clause introduced by toen. In Dutch subordinate clauses the finite verb moves to the very end:
“… Toen het onweer begon.”

Why does the second part of the sentence start with had instead of the subject het publiek?

After a subordinate clause ([Toen het onweer begon]), the main clause follows. Dutch main clauses follow the Verb-Second (V2) rule: the finite verb must be the second constituent. Here the entire subordinate clause counts as the first constituent, so had appears immediately after it, before the subject het publiek.

Why is the past perfect (had verlaten) used instead of the simple past (verliet)?

The past perfect (pluperfect) indicates that one past action was completed before another past action. In this sentence, leaving the cinema happened before the thunderstorm started. Using simple past (verliet) would only state that they left, without clarifying the sequence of events.

Why is the auxiliary verb had singular and not hadden?

Because the subject het publiek is a singular collective noun in Dutch. Collective nouns like publiek take a singular verb form:
“Het publiek had” not “hadden.”

What does al mean here, and why is it placed before verlaten?

Al means “already.” In Dutch, time-adverbs such as al typically go between the auxiliary verb (had) and the past participle (verlaten):
“… had het publiek de bioscoop al verlaten.”

Why is the word for “cinema” de bioscoop and not het bioscoop?

Because bioscoop is a common-gender noun in Dutch. Common-gender nouns take the definite article de, while only neuter nouns take het.

Why does verlaten use hebben (had) and not zijn?

In Dutch, most transitive verbs (verbs that take a direct object) form their compound tenses with hebben. Verlaten (“to leave [something]”) is transitive (you leave a place), so it uses hebben rather than zijn, which is reserved for certain intransitive/movement verbs.

Can I swap the clauses and say Het publiek had de bioscoop al verlaten toen het onweer begon?

Yes. You can front the main clause:
“Het publiek had de bioscoop al verlaten toen het onweer begon.”
Since Het publiek is now the first element, had follows in second position, and the subordinate clause toen het onweer begon comes last.