Un atasco en la avenida retrasó el autobús y llegué a clase con cinco minutos de retraso.

Because un introduces it as a traffic jam, not the traffic jam.

A native speaker would use un atasco when:

• the traffic jam is being mentioned for the first time
• it is not a specific one already known to the listener

If both speakers already knew which traffic jam they were talking about, el atasco could be possible, but in this sentence un atasco is the natural choice.

In this sentence, yes: atasco means traffic jam.

In Spanish from Spain, atasco is a very common everyday word for road congestion. In other contexts, it can also mean a blockage or jam more generally, such as a blocked pipe.

A learner should know:

• atasco = very common in Spain for traffic jam
• In some parts of Latin America, learners may hear embotellamiento or trancón instead

Because en la avenida tells you where the traffic jam happened: on the avenue / in the avenue area.

• en = location
• de would usually suggest possession, origin, or a descriptive relationship, not simple location

So:

• Un atasco en la avenida = A traffic jam on the avenue
• Un atasco de la avenida would sound unnatural here

Yes. Here, retrasó is a transitive verb, and el autobús is the thing being delayed.

Structure:

• Un atasco = subject
• retrasó = verb
• el autobús = direct object

So literally:

• A traffic jam delayed the bus

This is very natural Spanish.

Because retrasó el autobús and se retrasó el autobús mean slightly different things.

• Un atasco retrasó el autobús = A traffic jam delayed the bus
  • the cause is stated directly
• El autobús se retrasó = The bus was delayed / The bus got delayed
  • this focuses more on what happened to the bus, not on the external cause

Since the sentence explicitly names the cause, retrasó el autobús is the best choice.

Because the speaker is talking about a completed event in the past: they arrived late to class on that occasion.

The preterite llegué is used for:

• a finished action
• a specific event

So:

• llegué a clase = I arrived at class

If you used the imperfect, it would suggest background, repetition, or an ongoing action, which is not the point here.

Because ir/llegar a clase is a very common expression meaning to go/arrive to class in a general sense.

In Spanish, some nouns are often used without an article when talking about the activity or destination in a general way:

• ir a clase
• estar en clase
• volver de clase

Using la is possible in some contexts, but it would sound more specific, like a particular class session or classroom. Here the general expression a clase is the natural one.

Because con cinco minutos de retraso is a very natural and precise way to say five minutes late.

Literally, it means:

• with five minutes of delay

Spanish often expresses lateness this way:

• Llegó con diez minutos de retraso
• Salió con media hora de retraso

You can also say:

• Llegué cinco minutos tarde

Both are correct, but con cinco minutos de retraso sounds a bit more formal or measured.

Yes, they come from the same root: retras-, related to delay or lateness.

But they are different parts of speech:

• retrasó = verb
  
  • it delayed
• retraso = noun
  
  • delay

So:

• Un atasco ... retrasó el autobús = A traffic jam delayed the bus
• con cinco minutos de retraso = with five minutes of delay

This kind of repetition is completely normal in Spanish.

Because the personal a is normally used with people and sometimes pets or personified beings, not with ordinary objects like el autobús.

Compare:

• Vi a María = I saw María
• Vi el autobús = I saw the bus

So in:

• retrasó el autobús

there is no a, because el autobús is just a thing, not a person.

Y simply connects two related past events:

• Un atasco en la avenida retrasó el autobús
• llegué a clase con cinco minutos de retraso

Yes, you could split them:

• Un atasco en la avenida retrasó el autobús. Llegué a clase con cinco minutos de retraso.

But using y makes the connection smoother and shows that the second event is a consequence of the first.

Each accent mark is there for a different reason.

• retrasó

  • the accent shows stress on the final syllable
  • it also helps distinguish the past tense form retrasó from other forms

• llegué

  • the accent marks the stressed final syllable
  • the spelling gué is needed so the g keeps the hard sound before e

• autobús

  • the written accent shows the stress falls on the last syllable

Accent marks in Spanish are not optional; they are part of the correct spelling.

The given word order is very natural, but Spanish does allow some flexibility.

Normal version:

• Un atasco en la avenida retrasó el autobús y llegué a clase con cinco minutos de retraso.

Possible variations:

• Llegué a clase con cinco minutos de retraso porque un atasco en la avenida retrasó el autobús.
• El autobús se retrasó por un atasco en la avenida y llegué a clase con cinco minutos de retraso.

However, the original sentence is clear and idiomatic:

• cause first
• result second

That makes it a very good model sentence for learners.