Qual foi o motivo do atraso ontem à noite?

Qual is natural when you are asking someone to identify a specific reason: Qual foi o motivo...? = What was the reason...?

In Portuguese, qual often sounds a bit more structured or specific than o que in this kind of sentence.

• Qual foi o motivo do atraso? = What was the reason for the delay?
• O que causou o atraso? = What caused the delay?

Both are possible, but they are built differently.
This sentence uses the noun motivo (reason), so qual fits very well.

Foi is the third-person singular preterite of ser:

• ser = to be
• foi = was

So Qual foi o motivo...? literally means What was the reason...?

It is in the past because the speaker is asking about a past event: the delay happened ontem à noite (last night).

Do is a contraction of:

• de
  • o = do

So:

• o motivo de o atraso becomes o motivo do atraso

In Portuguese, motivo commonly connects to what it refers to with de:

• o motivo do atraso = the reason for the delay
• o motivo da mudança = the reason for the change

This structure is very common.

Portuguese uses articles much more often than English does.

So where English says:

• What was the reason...?

Portuguese naturally says:

• Qual foi o motivo...?

Using o before motivo is normal and expected here.
Leaving it out would sound wrong in standard Portuguese.

Ontem à noite means last night or more literally yesterday at night / yesterday evening.

Breakdown:

• ontem = yesterday
• à noite = at night / in the evening

Together, they refer to the nighttime period of yesterday, which English usually expresses as last night.

The à is the result of crase, which happens when:

• the preposition a
• combines with the feminine article a

So:

• a + a noite = à noite

This is a fixed and very common expression:

• de manhã = in the morning
• à tarde = in the afternoon
• à noite = at night / in the evening

The accent is important because a noite and à noite are not the same thing in writing.

Yes, it sounds a bit formal or careful, though still perfectly natural.

Qual foi o motivo do atraso ontem à noite? sounds like something you might hear in:

• customer service
• work situations
• polite conversation
• written communication

More conversational alternatives could be:

• Por que houve atraso ontem à noite? = Why was there a delay last night?
• O que causou o atraso ontem à noite? = What caused the delay last night?
• Por que atrasou ontem à noite? = Why was it delayed last night? / Why did it run late last night?

Yes. Razão and motivo are both possible here.

• Qual foi o motivo do atraso...?
• Qual foi a razão do atraso...?

Both mean basically the same thing: What was the reason for the delay?

However, motivo is often a little more common and neutral in everyday use.
Razão can sound slightly more formal depending on context.

The structure is:

• Qual = what/which
• foi = was
• o motivo = the reason
• do atraso = for the delay / of the delay
• ontem à noite = last night

So the pattern is roughly:

Question word + verb + noun phrase + time expression

Portuguese often keeps this kind of order in questions without needing a helper verb like English did.

English:

• What was the reason for the delay last night?

Portuguese:

• Qual foi o motivo do atraso ontem à noite?

It refers to the time of the delayed event. In practice, the meaning is:

• What was the reason for the delay that happened last night?

The time expression usually attaches naturally to the whole event.
Portuguese does not need to repeat extra words to make that clear.

If you wanted to make it even more explicit, you could say:

• Qual foi o motivo do atraso de ontem à noite?

That is also possible, but the original sentence already sounds natural and clear.

A broad Brazilian-style pronunciation would sound roughly like:

kwal foy oo mo-CHEE-voo doo ah-TRAH-zoo oh-CHEN ah NOY-chee

A few helpful notes:

• qual often sounds close to kwau/kwal
• foi sounds like foy
• motivo has stress on ti: mo-TI-vo
• atraso has stress on tra: a-TRA-so
• noite has stress on noi: NOI-te
• in many Brazilian accents, unstressed final -e sounds like ee

This is only an approximation, but it can help you hear the rhythm.