El portero deja un aviso en el ascensor.

Not in this sentence.

In Spanish, portero can mean different things depending on context:

• goalkeeper in sports
• doorman / concierge / building caretaker in a building

Here, because the sentence talks about leaving a notice in the elevator, el portero means the building porter / concierge / caretaker.

In Spain, this is a very common meaning of portero.

Spanish usually uses the definite article more often than English.

So el portero means:

• the porter
• or sometimes the building porter, referring to the person in that role

Even when English might say something like the concierge or just concierge in a general context, Spanish often keeps the article.

Deja is the third-person singular present tense of dejar.

So it means:

• he leaves
• she leaves
• it leaves
• or sometimes does leave / is leaving, depending on context

Here, with el portero, it means the porter leaves.

That is a very common question, because dejar has several meanings.

Common meanings of dejar include:

• to leave something somewhere
• to let / allow
• to stop / give up in some contexts

In this sentence, deja un aviso en el ascensor means:

• leaves a notice in the elevator

So here dejar means to leave something behind or place it somewhere.

Aviso often means:

• notice
• announcement
• warning
• message

In this sentence, un aviso is most naturally understood as a notice or a message.

It does not necessarily mean a strong danger warning. The exact nuance depends on context.

For example, in a building, un aviso could be:

• a note for residents
• a maintenance notice
• an announcement about repairs

Because un means a/an, so the sentence is talking about a notice, not a specific previously mentioned one.

Compare:

• deja un aviso = leaves a notice
• deja el aviso = leaves the notice / leaves the specific notice

So un introduces it as something new or unspecified.

En usually means in, on, or at, depending on context.

Here:

• en el ascensor = in the elevator

Spanish uses en very naturally for location. So the porter is leaving the notice in the elevator.

Spanish usually needs an article with singular countable nouns in this kind of sentence.

So:

• en el ascensor = in the elevator

Without the article, en ascensor would not sound natural here.

Also, el ascensor is masculine because ascensor is a masculine noun.

Yes. In Spain, ascensor is the standard word for elevator / lift.

You may also hear:

• elevador in some varieties of Spanish, but this is less usual in Spain for a normal building elevator
• In British English, the translation is often lift

So for Spain Spanish, ascensor is exactly the word learners should know.

Yes, but the original order is the most neutral.

Normal order:

• El portero deja un aviso en el ascensor.

You could also say things like:

• En el ascensor, el portero deja un aviso.

That puts more focus on where the notice is left.

Spanish word order is fairly flexible, but the basic subject + verb + object + place order is the most straightforward here.

The subject is el portero.

Spanish verbs change form depending on the subject, and deja is a third-person singular form. That matches:

• él deja
• ella deja
• el portero deja

So the porter is the one performing the action.

Yes, Spanish can omit the subject, because the verb form often makes it clear.

So:

• El portero deja un aviso en el ascensor.
• Deja un aviso en el ascensor.

Both are possible.

The full version with el portero is used when you want to:

• identify who does the action
• emphasize the subject
• avoid ambiguity

Yes, a few useful things:

• portero in Spain often means building concierge / caretaker, not just goalkeeper
• dejar often means to leave something somewhere
• aviso can be broader than just warning; it may be a notice or message
• ascensor is the usual Spain Spanish word for elevator
• Spanish uses articles very often: el portero, un aviso, el ascensor

So although the sentence is short, it contains several very common Spanish patterns.