Learn Hardcore Dutch: De teleurstelling was groot toen het concert werd afgelast. - The disappointment was great when the concert was cancelled.

Werd is the past tense of worden (to become/be done).
Afgelast is the past participle of afgelasten.
Together they form a passive construction, which shifts the focus away from who cancelled the concert to the fact that the concert was cancelled.

Could we also say is afgelast instead of werd afgelast? What’s the difference?

Yes, you can say is afgelast, but that’s the perfect passive (present perfect) and emphasizes the current result (“the concert is now cancelled”).

Werden + past participle (werd afgelast) = simple past passive, used in narratives to tell about a past event.
Zijn + past participle (is afgelast) = perfect passive, often used to describe a state resulting from a past action.

Why is toen used to introduce the time clause?

Toen is the subordinating conjunction for a single event in the past, meaning “when” (at the moment that).

Use toen for specific past moments (toen ik klein was…).
Use als for repeated/habitual events or conditions (als ik tijd heb…).
Use nadat when you want to say “after” something happened (nadat het concert was afgelast…).

Why does the verb go to the end in toen het concert werd afgelast?

Because toen is a subordinating conjunction, Dutch grammar requires the finite verb to move to the end of the clause. Structure:
toen + subject + (other elements) + verb (werd) + past participle (afgelast).

Why is it was groot and not was grote?

Adjectives in predicative position (after linking verbs like zijn = to be) remain uninflected in Dutch. You use the base form:
– Predicative: De teleurstelling was groot.
– Attributive (before a noun), you would say: de grote teleurstelling (adjective takes -e).

Why is the article de used before teleurstelling? What gender is it?

Teleurstelling is a common‐gender noun in Dutch, so it takes de, not het. Modern Dutch has two article classes:

de for common‐gender nouns (both masculine and feminine historically).
het for neuter nouns.
There’s no other gender marking, so ‘de teleurstelling’ is just something to memorize.

Should there be a comma before toen het concert werd afgelast?

In Dutch, the comma before a subordinate clause following the main clause is optional. You can write:
De teleurstelling was groot toen het concert werd afgelast
or
De teleurstelling was groot, toen het concert werd afgelast
Adding the comma can improve readability, especially in longer sentences, but it isn’t required.

Could we start the sentence with the subordinate clause? What changes?

Yes. Fronting the time clause is perfectly fine:
Toen het concert werd afgelast, was de teleurstelling groot.
When you begin with a subordinate clause, you:

Keep the verb at the end of that clause.
Place a comma.
In the main clause that follows, use the verb‐second rule (the conjugated verb was appears in second position).