La portinaia saluta la nuova inquilina con un sorriso.

Because they refer to female people, and in Italian nouns for people usually change form to match gender.

• portinaio = male concierge/doorman/caretaker

• portinaia = female concierge/doorkeeper/caretaker

• inquilino = male tenant
• inquilina = female tenant

That is why the sentence uses the feminine forms portinaia and inquilina.

Italian uses definite articles very often, more often than English does in some cases. Here both noun phrases are specific:

• La portinaia = the concierge/doorkeeper
• la nuova inquilina = the new tenant

The first la marks the subject, and the second la is part of the direct object. Italian does not drop the article just because the noun comes after the verb.

So the structure is:

• La portinaia = subject
• saluta = verb
• la nuova inquilina = direct object

Adjectives must agree with the noun they describe in gender and number.

Since inquilina is feminine singular, the adjective must also be feminine singular:

• masculine singular: nuovo inquilino
• feminine singular: nuova inquilina
• masculine plural: nuovi inquilini
• feminine plural: nuove inquiline

So nuova matches inquilina.

In Italian, many adjectives can come either before or after the noun, but some positions are more natural depending on style and meaning.

With common adjectives like nuovo, putting it before the noun is very normal:

• la nuova inquilina

This sounds natural and usually means the new tenant in a general, identifying sense.

You can also find l’inquilina nuova, but that often sounds more contrastive, as if you are distinguishing her from another tenant:

• la nuova inquilina = the new tenant
• l’inquilina nuova = the tenant who is new / the new one among tenants

So in this sentence, la nuova inquilina is the most natural choice.

Saluta is the third-person singular present tense of salutare.

Conjugation:

• io saluto = I greet
• tu saluti = you greet
• lui/lei saluta = he/she greets
• noi salutiamo = we greet
• voi salutate = you all greet
• loro salutano = they greet

Here the subject is La portinaia, which is singular, so the verb must be singular too:

• La portinaia saluta... = The concierge greets...

Italian usually leaves out subject pronouns when they are not needed, because the verb ending already shows the person and number.

So instead of saying:

• Lei saluta la nuova inquilina

Italian very naturally says:

• La portinaia saluta la nuova inquilina

The noun La portinaia already tells you who the subject is, so adding lei would be unnecessary unless you want emphasis.

Con un sorriso is a prepositional phrase introduced by con = with.

It tells you how the greeting happens, so it expresses manner:

• saluta ... con un sorriso = greets ... with a smile

Breakdown:

• con = with
• un = a
• sorriso = smile

It is not the direct object. The direct object is la nuova inquilina.
Con un sorriso just adds extra information about the way she greets her.

Because the sentence means with a smile, not with the smile.

Italian uses the indefinite article un when the thing is non-specific or introduced for the first time:

• un sorriso = a smile

If you said con il sorriso, that would usually suggest a specific smile already known from context, which is not the idea here.

Yes, often it can.

Salutare basically means to greet / to address politely / to say hello to / to say goodbye to, depending on context.

Examples:

• Saluto i vicini ogni mattina. = I greet the neighbors every morning.
• Ho salutato tutti prima di uscire. = I said goodbye to everyone before leaving.

In this sentence, the meaning is clearly greets.

Italian word order helps here:

• La portinaia comes before the verb, so it is naturally read as the subject.
• saluta is the verb.
• la nuova inquilina comes after the verb, so it is naturally the direct object.

Also, semantically it makes sense: the concierge is the one doing the greeting, and the new tenant is the person receiving it.

So:

• doer of the action = La portinaia
• receiver of the action = la nuova inquilina

Yes, Italian word order is fairly flexible, but the basic order here is the most neutral and natural:

• La portinaia saluta la nuova inquilina con un sorriso.

Other orders are possible for emphasis, for example:

• Con un sorriso, la portinaia saluta la nuova inquilina.
• La nuova inquilina la saluta la portinaia... would be much less neutral and may require special context.

For a learner, the original order is the one to remember: subject + verb + object + extra phrase.

Everything would have to agree in gender and number.

Masculine singular:

• Il portinaio saluta il nuovo inquilino con un sorriso.

Feminine plural:

• Le portinaie salutano le nuove inquiline con un sorriso.

Masculine plural:

• I portinai salutano i nuovi inquilini con un sorriso.

This is a good example of how articles, nouns, adjectives, and verbs all work together in Italian.