Ho dovuto disdire l’appuntamento dal dentista, perché il treno era in ritardo.

Ho dovuto is the passato prossimo of dovere, which means to have to / must.

• ho = I have
• dovuto = past participle of dovere

So ho dovuto disdire literally means I have had to cancel, but in natural English it is usually just I had to cancel.

Italian often uses the passato prossimo where English uses the simple past.

Disdire is very commonly used for canceling:

• appointments
• reservations
• bookings
• subscriptions

So disdire l’appuntamento is a very natural way to say cancel the appointment.

Cancellare also means to cancel, but it is often more general. For an appointment, disdire sounds especially appropriate.

It implies that the appointment was actually called off, not just that there was an intention to do so.

Because the sentence says ho dovuto disdire, it presents the cancellation as a completed action: the speaker ended up having to cancel it.

So the idea is not just I was supposed to cancel or I needed to cancel, but rather I had to cancel, and I did.

Because appuntamento begins with a vowel.

In Italian, the singular definite article before a vowel is often shortened to l’:

• il libro
• l’appuntamento
• l’amico

So l’appuntamento simply means the appointment.

Dal is a contraction of:

• da + il = dal

With professions such as dentista, medico, parrucchiere, Italian often uses da to mean at/to the place of that professional.

So:

• andare dal dentista = to go to the dentist
• un appuntamento dal dentista = an appointment at the dentist’s / with the dentist

Here dal dentista is very natural because it refers to the dentist’s office or the visit there.

Yes. L’appuntamento con il dentista is also correct.

There is a slight nuance:

• dal dentista emphasizes the visit/location
• con il dentista emphasizes the person you have the appointment with

In many everyday situations, both work well.

Perché here means because and introduces the reason for canceling the appointment.

So the structure is:

• main clause: Ho dovuto disdire l’appuntamento dal dentista
• reason clause: perché il treno era in ritardo

The accent on é is part of the correct spelling: perché.

Era is the imperfetto of essere.

Italian often uses the imperfetto for background situations, ongoing states, or circumstances that explain another event. In this sentence, the train being late is the situation that caused the cancellation.

So:

• il treno era in ritardo = the train was late

Using è stato in ritardo would sound more like a completed fact being reported on its own, and it is less natural in this kind of explanatory sentence.

In ritardo is a fixed expression meaning late.

So:

• essere in ritardo = to be late

Italian does not use an article here. You simply say:

• Sono in ritardo = I’m late
• Il treno era in ritardo = The train was late

You can also add how late:

• Il treno era in ritardo di venti minuti = The train was twenty minutes late

Not exactly.

• Ho dovuto disdire = I had to cancel
  This sounds like a completed event and strongly suggests the cancellation actually happened.

• Dovevo disdire = I had to / was supposed to cancel
  This can sound more like background information, an obligation, or something that was pending, without focusing as strongly on completion.

So in this sentence, ho dovuto disdire is the better choice because the speaker is talking about a specific finished action.

It can be rearranged.

For example, you could also say:

Perché il treno era in ritardo, ho dovuto disdire l’appuntamento dal dentista.

That means the same thing. Italian word order is fairly flexible, especially with clauses introduced by perché.

The original version sounds completely natural and probably a bit more neutral in everyday speech.