Learn Hardcore Spanish (Spain): Anoche llegué tarde a casa por el atasco. - Last night I arrived home late because of the traffic jam.

llegué can only mean “I arrived” (1st person singular, preterite).
Because it’s clear from the ending -é that the subject is yo, you don’t need to say yo.

You can say Yo anoche llegué tarde a casa por el atasco, but that adds emphasis, like “I (as opposed to someone else) arrived late…”.
The neutral, most natural version simply omits yo: Anoche llegué…

Why is llegué in the preterite tense and not in the imperfect?

The preterite is used for single, completed events in the past. Arriving home late is one completed action that happened once last night, so llegué (preterite) is correct.

Anoche llegué tarde a casa… = Last night I (once) arrived home late…

If you used the imperfect (llegaba), it would suggest a repeated or ongoing situation, or background description:

Cuando vivía más lejos, siempre llegaba tarde a casa por el atasco.
When I lived farther away, I would always arrive home late because of traffic.

So here, a single event → preterite llegué.

Why is it a casa and not en casa?

Because the verb llegar involves movement to a place, and Spanish normally uses a with verbs of movement.

llegar a casa = to arrive (to) home
ir a casa = to go (to) home

en casa means “at home”, not “to home”:

Estoy en casa. = I am at home.
Trabajo en casa. = I work at home.

So with llegar, a casa is the natural choice.

Why is it a casa and not a la casa?

In Spanish, when you talk about going to your own home, the normal phrase is simply a casa (no article, no possessive):

Vuelvo a casa. = I’m going back home.
Llego tarde a casa. = I arrive home late.

You would use a la casa when you are talking about a specific house, not necessarily your own, or when you need to distinguish it from home in general:

Fuimos a la casa de mis abuelos. = We went to my grandparents’ house.
Entraron a la casa equivocada. = They went into the wrong house.

So a casa here is understood as “(to) my home”.

Could I say a mi casa instead of a casa?

Yes, you can say a mi casa, and it is grammatically correct:

Anoche llegué tarde a mi casa por el atasco.

However, in everyday speech, Spanish speakers usually drop mi when talking about their own home, unless they want to emphasize or contrast it:

Neutral: Llego tarde a casa.
Emphasis/contrast: Llego tarde a mi casa, pero tú llegas temprano a la tuya.
(I arrive late at my house, but you arrive early at yours.)

What exactly does tarde mean here? Does it mean “afternoon” or “late”?

In this sentence, tarde is an adverb meaning “late”:

Llegué tarde = I arrived late.

tarde can also be a noun meaning “afternoon”:

Por la tarde = in the afternoon.

You know which meaning is intended from the structure:

llegar tarde is a fixed expression: to arrive late.
When used with an article (la tarde) it usually means afternoon:
por la tarde, esta tarde.

Why is it por el atasco and not para el atasco?

Here por introduces the cause or reason:

por el atasco ≈ because of the traffic jam

por + noun is often used for causes:

Llegamos tarde por la lluvia. = We were late because of the rain.
No salí por el frío. = I didn’t go out because of the cold.

para is used for purpose / destination / goal, not cause.
Llegué tarde para el atasco does not mean “because of the traffic jam” and sounds wrong here.

You could also express the cause with a full clause:

Llegué tarde a casa porque había un atasco.
(because there was a traffic jam)

What does atasco mean, and is it the usual word in Spain?

atasco literally means “a blockage” or “jam”. In traffic, it is a traffic jam.

In Spain, un atasco (de tráfico) is a very common way to say “traffic jam”:

Había un atasco enorme en la autopista.

Other options you might hear in Spain:

embotellamiento – also “traffic jam”, a bit more formal/technical.
había mucho tráfico – “there was a lot of traffic” (not exactly the same, but often used similarly).

atasco can also refer to other types of blockage:

un atasco en el fregadero = a blockage in the sink.

Why is it el atasco and not un atasco?

Both are grammatically possible, but they sound slightly different.

por el atasco = because of the traffic jam
→ Sounds like a specific, definite jam (for example, the one on your usual route, or the one both speaker and listener know about).
por un atasco = because of a traffic jam
→ Introduces it as just “some traffic jam”, not previously identified.

In everyday conversation about why you were late, por el atasco is very natural in Spain, as if you and the listener both understand “the usual jam” or “the jam on that road tonight”.

Could I say porque el atasco instead of por el atasco?

No, porque must introduce a clause (a whole sentence), not just a noun.

Correct:

Llegué tarde a casa porque había un atasco.
(because there was a traffic jam)

Incorrect:

✗ Llegué tarde a casa porque el atasco.

If you only want to use a noun after the connector, use por or another preposition:

por el atasco
a causa del atasco
debido al atasco

Can I use the present perfect and say Anoche he llegado tarde a casa?

No, that combination is not natural in standard Spanish.

The present perfect (he llegado) in Spanish is normally used with time expressions that include the present or are connected to “now”, especially in Spain:

Hoy he llegado tarde. = Today I have arrived late.
Esta semana he llegado tarde tres veces.

But anoche (last night) refers to a finished time period, completely separated from the present. For that, Spanish uses the preterite:

Anoche llegué tarde a casa. ✔

So Anoche he llegado tarde… sounds wrong to most native speakers.

Can I move anoche to another place in the sentence?

Yes. Several word orders are possible, all grammatical, with small differences in emphasis. For example:

Anoche llegué tarde a casa por el atasco.
(Time at the beginning – very common.)
Llegué tarde a casa anoche por el atasco.
(Time in the middle, still normal.)
Llegué tarde a casa por el atasco anoche.
(Possible, but can sound a bit heavier; focus may feel more on when the cause happened.)

In everyday speech, the two most natural are:

Anoche llegué tarde a casa por el atasco.
Anoche llegué tarde a casa porque había un atasco.

Could I say something like Anoche, por el atasco, llegué tarde a casa?

Yes, that is also correct. Placing por el atasco earlier and setting it off with commas gives it a bit more emphasis or a slightly more careful / written tone:

Anoche, por el atasco, llegué tarde a casa.

Meaning is the same; you are just highlighting the cause more explicitly. In fast, everyday speech, most people would keep the original order:

Anoche llegué tarde a casa por el atasco.