Learn Hardcore German: Wir müssen das Konzert absagen.

Because with modal verbs in a main clause, the conjugated modal takes the second position and the main verb goes to the end in the infinitive. In Wir müssen das Konzert absagen, müssen is in V2, and absagen (infinitive) moves to the end. Without a modal, you’d say: Wir sagen das Konzert ab.

Why isn’t absagen split into sagen … ab here?

Separable-prefix verbs (like ab-sagen) split only when they are the finite verb. With a modal, you use the infinitive, which stays together: Wir müssen das Konzert absagen. But: Wir sagen das Konzert ab.

Why is it das Konzert and not den Konzert?

Konzert is a neuter noun (das Konzert). As a direct object (accusative), neuter stays das. Only masculine nouns change to den in the accusative.

Does müssen mean “must” or “have to”?

Both; müssen expresses necessity/obligation. It’s stronger than sollen (should) and unrelated to permission (dürfen = may). Here it means “we have to/must.”

How do I say “don’t have to” vs “must not”?

Wir müssen das Konzert nicht absagen. = We don’t have to cancel the concert.
Wir dürfen das Konzert nicht absagen. = We must not/may not cancel the concert (it’s not allowed).
Note that nicht müssen ≠ “must not.”

Where can I put leider?

Common and natural options:

Wir müssen das Konzert leider absagen.
Leider müssen wir das Konzert absagen.
Wir müssen leider das Konzert absagen. All are fine; earlier placement of leider can make the regret more prominent.

Can I use other verbs like stornieren or annulieren?

For events, absagen is the standard. Alternatives:

Das Konzert fällt aus. (The concert is canceled; impersonal/passive-like.)
Wir müssen den Auftritt abblasen. (Colloquial “call off”.)
stornieren is for bookings/reservations (tickets, hotel), not typically whole concerts.
annullieren is formal/administrative and uncommon in everyday speech for concerts.

What cases does absagen take?

Two patterns:

Cancel something (accusative): Wir müssen das Konzert absagen.
Decline/cancel on someone (dative): Ich muss dir absagen. / Wir mussten den Fans absagen. You can also say with both (more common with appointments): Wir müssen ihm den Termin absagen.

How do I say it in the past?

Simple past (common in writing): Wir mussten das Konzert absagen.
Conversational perfect with a modal (double infinitive): Wir haben das Konzert absagen müssen. If you canceled without a modal: Wir haben das Konzert abgesagt.

How do I form the yes/no question?

Invert subject and modal: Müssen wir das Konzert absagen?

What happens in a subordinate clause?

Both verbs go to the end, with the modal last: …, weil wir das Konzert absagen müssen.

Can I replace das Konzert with a pronoun?

Yes: Wir müssen es absagen. With pronouns, the accusative pronoun (es) typically comes before the infinitive cluster and before any dative noun: Wir müssen es dem Veranstalter absagen (more natural with appointments: Wir müssen ihm den Termin absagen.)

Do I need zu before absagen?

Not after a modal. You say müssen … absagen (no zu). With non-modals that govern a zu-infinitive, insert zu between prefix and stem: Wir versuchen, das Konzert abzusagen.

Anything to watch out for in spelling/pronunciation?

müssen has ü (round your lips; like French u) and ss (not ß) because the vowel is short.
The stress in absagen is on the prefix: AB-sagen when split: wir sagen … AB.

How can I make it sound softer/more polite?

Add softeners or framing:

Leider müssen wir das Konzert absagen.
Wir werden das Konzert leider absagen müssen.
Formal: Wir sehen uns leider gezwungen, das Konzert abzusagen.

Is Konzert capitalized?

Yes. All nouns in German are capitalized: das Konzert.