El cartero dejó el paquete en la puerta porque nadie estaba en casa.

In Spanish, articles like el, la, los, las are used more often than in English.

So el cartero means the mailman / the postman. In a full sentence, Spanish usually sounds more natural with the article here.

• El cartero dejó el paquete... = The mailman left the package...

If you said just cartero, it would sound incomplete in this context.

The accent mark in dejó shows both pronunciation and meaning.

• dejó = he/she/it left (preterite tense)
• dejo = I leave (present tense)

So the accent helps distinguish different verb forms.

Here:

• dejó = he left

The stress falls on the last syllable: de-JÓ.

Dejó is the preterite, which is used for a completed action in the past.

In this sentence, the mailman performed one finished action:

• left the package

So dejó is the natural choice.

By contrast, dejaba is the imperfect, which often suggests a repeated, ongoing, or background action:

• El cartero dejaba el paquete... could suggest something habitual or descriptive, like The mailman used to leave the package...

Here we want a single completed event, so dejó is correct.

This is a very common question because both forms mean was in English.

Spanish uses:

• imperfect (estaba) for background situations, states, or ongoing circumstances
• preterite (estuvo) for completed events or bounded situations

In this sentence, nadie estaba en casa describes the situation at that time: nobody was at home. It gives background explaining why the package was left at the door.

So:

• porque nadie estaba en casa = because nobody was home

Using estuvo would sound more like referring to a completed stretch of being at home/not at home, which is not the idea here.

Because nadie is grammatically singular in Spanish, even though in English nobody refers to zero people.

So the verb must also be singular:

• nadie estaba = nobody was

Not:

• nadie estaban

Other examples:

• Nadie sabe = Nobody knows
• Nadie vino = Nobody came

In Spanish, en casa is a very common expression meaning at home.

• estar en casa = to be at home

It usually does not need an article.

By contrast, en la casa usually means in the house or refers to a specific house as a physical place.

So:

• nadie estaba en casa = nobody was home
• nadie estaba en la casa = more like nobody was in the house

Both are possible, but en casa is the idiomatic choice for at home.

Because en is used here for location: the package was left at/on/by the door.

• dejó el paquete en la puerta = left the package at the door

The preposition a is more often used for direction or movement toward something:

• fue a la puerta = he went to the door

So:

• en la puerta = location
• a la puerta = direction toward the door

These two are different in Spanish:

• porque = because
• por qué = why

In your sentence, the second part gives the reason:

• porque nadie estaba en casa = because nobody was home

So porque is correct.

Compare:

• ¿Por qué dejó el paquete en la puerta? = Why did he leave the package at the door?
• Lo dejó en la puerta porque nadie estaba en casa. = He left it at the door because nobody was home.

Spanish often omits subject pronouns when they are not needed, because the verb form already tells you who the subject is.

• dejó already tells us it is he/she/it left

Also, the noun el cartero is already there, so adding él would be unnecessary:

• El cartero dejó el paquete...

Spanish usually avoids repeating the subject unless you want emphasis or contrast.

Yes. Spanish allows some flexibility in word order.

You could say:

• El cartero dejó el paquete en la puerta porque nadie estaba en casa.
• Porque nadie estaba en casa, el cartero dejó el paquete en la puerta.

Both are grammatical.

The original version is more neutral and natural in ordinary narration. Starting with porque... puts more focus on the reason.

For the same reason as el cartero: Spanish often uses articles where English might or might not.

• el paquete = the package

It refers to a specific package in the situation, so the definite article is natural.

Saying just dejó paquete would be incorrect here. A noun like paquete normally needs a determiner, such as:

• el paquete = the package
• un paquete = a package
• mi paquete = my package

Yes. In this sentence, dejar means to leave in the sense of to leave something somewhere.

• dejar algo en algún sitio = to leave something in/at some place

So:

• dejó el paquete en la puerta = he left the package at the door

Be careful: English leave has several meanings, and Spanish may use different verbs depending on the situation. For example:

• leave something somewhere → dejar
• leave a place → often salir de or irse de

Example:

• Left the package at the door → Dejó el paquete en la puerta
• Left the house → Salió de la casa / Se fue de la casa