Te pido perdón por llegar tarde; no fue culpa mía, sino del atasco.

Because pedir perdón is a common way to say to ask for forgiveness / to apologize.

• te = to you
• pido = I ask
• perdón = forgiveness / pardon

So te pido perdón literally means I ask you for forgiveness.

It sounds a bit more explicit and heartfelt than just lo siento in many contexts.

Te is the indirect object pronoun, meaning to you.

Spanish often uses object pronouns where English would not:

• Te pido perdón = I ask you for forgiveness
• Le doy un libro = I give him/her a book

So te shows who the apology is directed to.

Because Spanish uses pedir for asking for something, while preguntar means to ask a question.

• pedir perdón = to ask for forgiveness
• preguntar = to ask in the sense of requesting information

So:

• Te pido perdón is correct.
• Te pregunto perdón is not natural Spanish.

After por, Spanish often uses the infinitive to express for doing something.

So:

• por llegar tarde = for arriving late
• literally, for to arrive late

This is very common in Spanish:

• Gracias por venir = Thanks for coming
• Perdón por molestar = Sorry for bothering
• Lo castigaron por mentir = They punished him for lying

Yes. Por haber llegado tarde is also correct.

The difference is:

• por llegar tarde = more natural and common in everyday speech
• por haber llegado tarde = emphasizes that the action was completed in the past

Both work here, but por llegar tarde sounds simpler and more idiomatic in normal conversation.

Because fue from ser presents it as a completed fact: it wasn’t my fault.

In this sentence, the speaker is referring to a specific situation that already happened, so fue fits well.

• No fue culpa mía = It wasn’t my fault
• No era culpa mía can also exist, but it sounds more descriptive or background-like, depending on context

For a single finished event, fue is the most natural choice.

Because in Spanish, after a noun like culpa, a possessive can come after the noun without an article:

• culpa mía = my fault
• amigo mío = a friend of mine
• casa suya = his/her house

You can also hear la culpa fue mía, where the article belongs to the noun in a different structure. But in no fue culpa mía, leaving out the article is normal and idiomatic.

Here sino means but rather.

It is used after a negative statement to correct or replace the first idea:

• No fue culpa mía, sino del atasco
  = It wasn’t my fault, but rather the traffic jam’s

Use sino when the second part replaces the first one.

Compare:

• No vino Juan, sino Pedro = It wasn’t Juan who came, but Pedro
• No fue culpa mía, sino del atasco

By contrast, pero just adds contrast and would not sound as precise here.

Del is the contraction of de + el.

• de el atasco → del atasco

So:

• culpa del atasco = the fault of the traffic jam

Spanish normally contracts de el into del, except with some proper names or special cases.

In Spain, atasco commonly means a traffic jam.

It can also mean a blockage or jam more generally, depending on context, but here it clearly refers to traffic.

For example:

• Hay un atasco enorme en la autopista = There’s a huge traffic jam on the motorway

This is a very common word in Spain.

Not in this sentence.

Here the structure is:

• no fue culpa mía, sino del atasco
• literally: it was not my fault, but the traffic jam’s

So de is needed because it connects culpa to its source or owner.

If you said por el atasco, that would be a different structure, for example:

• Llegué tarde por el atasco = I arrived late because of the traffic jam

So both are correct in Spanish, but they belong to different sentence patterns.

The semicolon links two closely related ideas:

1. Te pido perdón por llegar tarde
2. No fue culpa mía, sino del atasco

It works a bit like a strong comma or a soft full stop. It is not essential; a full stop would also be fine:

• Te pido perdón por llegar tarde. No fue culpa mía, sino del atasco.

The semicolon just makes the connection between the apology and the explanation feel tighter.

Yes, several alternatives are possible:

• Perdón por llegar tarde = Sorry for being late
• Lo siento por llegar tarde = I’m sorry for being late
• Te pido disculpas por llegar tarde = I apologize for being late

Compared with these:

• Perdón is short and common
• Lo siento is very common and natural
• Te pido perdón sounds more direct and sincere
• Te pido disculpas sounds a bit more formal

The sentence is fairly natural as it stands, but Spanish does allow some flexibility.

For example:

• Te pido perdón por llegar tarde; no fue culpa mía, sino del atasco.
• Por llegar tarde, te pido perdón; no fue culpa mía, sino del atasco.

The original order is the most straightforward and natural for everyday use. Spanish often moves parts of the sentence for emphasis, but learners should usually stick to the basic version first.