Sempre que eu chego tarde, meu pai pergunta por que eu demorei.

Sempre que means whenever or every time that. It shows a repeated situation, not just one specific moment.

So:

• Sempre que eu chego tarde... = Whenever I arrive late...
• Toda vez que eu chego tarde... = also very natural, with almost the same meaning

You can sometimes use quando, but sempre que is clearer if you want the idea of repetition.

• Quando eu chego tarde... can mean when/whenever I arrive late
• Sempre que eu chego tarde... strongly emphasizes every time

Because the sentence mixes:

• a habitual/repeated situation: Sempre que eu chego tarde
• a specific completed event that the father asks about each time: por que eu demorei

In other words:

• chego = what usually happens in general
• demorei = the delay on that particular occasion

So the logic is:

• Whenever I arrive late, my father asks why I was delayed / why I took so long.

That combination is very natural in Portuguese.

After sempre que, Portuguese can use either the indicative or the subjunctive, depending on meaning.

Here, chego is in the present indicative because the sentence describes a habit or something that regularly happens.

• Sempre que eu chego tarde... = whenever I arrive late / every time I arrive late

If you were talking about a future possibility, you would normally use the future subjunctive:

• Sempre que eu chegar tarde, meu pai vai perguntar...
  = Whenever I arrive late / If I happen to arrive late in the future, my father will ask...

So:

• chego = habitual, general
• chegar = future or hypothetical occasions

Because por que is used in a question meaning why or for what reason.

In this sentence, it introduces an indirect question:

• meu pai pergunta por que eu demorei
  = my father asks why I was delayed

The common forms are:

• por que = why / for what reason
• porque = because
• por quê = why? at the end of a question or before a pause
• porquê = a noun meaning the reason

Examples:

• Por que você chegou tarde? = Why did you arrive late?
• Cheguei tarde porque o ônibus atrasou. = I arrived late because the bus was late.
• Você chegou tarde por quê? = Why did you arrive late?
• Ninguém explicou o porquê. = No one explained the reason.

Demorei comes from demorar.

In this context, it usually means something like:

• I took a long time
• I was delayed
• I took too long

So por que eu demorei is very natural Portuguese, even if the smoothest English translation may be why I was late.

There is a small nuance:

• chegar tarde focuses on the result: to arrive late
• demorar focuses on the delay/time taken: to take too long / be delayed

So the father is basically asking for the reason for the delay.

Yes. Portuguese often drops subject pronouns when the verb ending already makes the subject clear.

So these are both natural:

• Sempre que eu chego tarde, meu pai pergunta por que eu demorei.
• Sempre que chego tarde, meu pai pergunta por que demorei.

The version with eu can sound a little more explicit or emphatic, but it is not required.

Both can exist, but meu pai is very natural in Brazilian Portuguese.

With possessives, Portuguese may use:

• meu pai
• o meu pai

In Brazil, both patterns appear, but with close family words like pai, mãe, irmão, the version without the article is especially common and very natural.

So:

• meu pai = natural
• o meu pai = also possible, depending on region, style, or emphasis

Because Sempre que eu chego tarde is an introductory clause placed before the main clause.

Structure:

• Sempre que eu chego tarde, = introductory time clause
• meu pai pergunta por que eu demorei. = main clause

In Portuguese, it is very common to separate that fronted clause with a comma.

So the comma helps show:

• first, the condition/time frame
• then, the main action

Here, tarde functions as an adverb meaning late.

• chegar tarde = to arrive late

It is not describing a noun; it is modifying the verb chegar.

Compare:

• uma hora tardia = a late hour (adjective-like idea)
• cheguei tarde = I arrived late (adverb)

So in this sentence, think of tarde simply as the adverb late.

Because Portuguese does not need the same kind of auxiliary + inversion pattern that English uses.

In English:

• Why was I late?
• Why did I take so long?

In Portuguese, the normal order is often kept:

• Por que eu demorei?

That is true in both direct and indirect questions.

So:

• Meu pai pergunta por que eu demorei.
• literally: My father asks why I took so long.

Portuguese does not need an equivalent of English do in this kind of question.

Not necessarily. Perguntar can take:

• the thing being asked
• and optionally the person being asked

Here, the sentence focuses on the content of the question:

• meu pai pergunta por que eu demorei
  = my father asks why I was delayed

If you want to make it explicit that he asks me, you can say:

• Meu pai me pergunta por que eu demorei.

That is also correct.

So:

• without me = he asks the question
• with me = he asks me the question

Because demorei refers to a specific completed delay.

• demorei = I was delayed / I took a long time on that occasion
• demorava = I used to take a long time / I was taking a long time / background or repeated past

In this sentence, the father is asking about the reason for the delay that just happened, so demorei fits best.

If you said por que eu demorava, it would usually suggest something more habitual or unfinished in the past, which does not match this situation as well.

That sentence sounds odd because the tenses do not match well.

• Sempre que eu chegar tarde points to future occasions
• meu pai pergunta is present habitual
• eu demorei points to a completed past event

A better future-oriented version would be:

• Sempre que eu chegar tarde, meu pai vai perguntar por que eu demorei.

Or, if you want to keep the original habitual meaning:

• Sempre que eu chego tarde, meu pai pergunta por que eu demorei.

So the original sentence is about a repeated present habit, while chegar would normally shift the meaning toward the future.