La propietaria quiere averiguar de dónde sale la humedad.

Spanish normally uses an article with a noun in cases like this. La propietaria means the female owner / landlady / property owner.

Saying just propietaria on its own would usually sound incomplete in a normal sentence unless it appeared in a special context, such as:

• a heading
• an answer to a question
• a label

So in a full sentence, La propietaria quiere... is the natural form.

Because the sentence is referring to a woman. Spanish nouns for people often change form depending on gender:

• el propietario = the male owner
• la propietaria = the female owner

The article changes too:

• el for masculine
• la for feminine

It means female owner, but in context it could be translated in different ways depending on the situation:

• owner
• landlady
• female property owner
• sometimes proprietor

If this is about a flat or building, la propietaria often suggests the woman who owns the property.

After querer when you want to say to want to do something, Spanish uses:

querer + infinitive

So:

• quiere = she wants
• averiguar = to find out

Together:

• quiere averiguar = she wants to find out

This is a very common structure:

• Quiero comer = I want to eat
• Quiere llamar = He/She wants to call

Averiguar means to find out, to investigate, or to determine.

In this sentence, it suggests trying to discover the source of the problem.

Compare:

• saber = to know
  
  • Quiere saber de dónde sale la humedad = She wants to know where the damp is coming from.
• averiguar = to find out / figure out
  
  • This emphasizes the process of investigating.
• descubrir = to discover
  
  • This often focuses more on the moment of discovery than on the investigation itself.

So averiguar is a very good choice here because it sounds like she is trying to work out the cause.

Because dónde is being used in an indirect question.

Even though the sentence is not a direct question like ¿De dónde sale la humedad?, it still contains the idea of a question inside it:

• quiere averiguar de dónde sale la humedad

This is an indirect question, and Spanish keeps the accent on interrogative words in indirect questions:

• qué
• quién
• cuándo
• cómo
• dónde

So:

• de dónde = from where / where ... from

Without the accent, donde is usually a relative word, as in:

• la casa donde vive = the house where he/she lives

Sale comes from salir, which usually means to go out, to come out, or to come from.

In this context, de dónde sale la humedad means something like:

• where the damp is coming from
• where the moisture is coming out of
• what the source of the damp is

It is a very natural way in Spanish to talk about the origin of a leak, smell, sound, problem, or substance.

For example:

• No sé de dónde sale el agua. = I don’t know where the water is coming from.
• Hay que ver de dónde sale el olor. = We need to see where the smell is coming from.

Sale is the third person singular present indicative of salir.

The subject is la humedad, which is singular, so the verb is singular too:

• la humedad sale

Some forms of salir:

• salgo = I go out / I come out
• sales = you go out
• sale = he/she/it comes out
• salen = they come out

Here it is not literally about going out. It means comes from / is coming from.

Because Spanish often uses the definite article with abstract or general nouns more often than English does.

Here la humedad means the damp / the moisture / the humidity, referring to the specific damp problem being discussed.

In context, la humedad often means:

• damp
• moisture
• dampness in a wall, ceiling, or building

So the article helps identify it as the known problem.

Not exactly in the everyday English sense.

Although humedad can translate as humidity, in housing or maintenance contexts it often means:

• damp
• moisture
• dampness

So in this sentence it probably refers to unwanted moisture in a building, such as a damp patch on a wall.

That is why a more natural English meaning in context is often damp rather than humidity.

Because the speaker is asking about a real source, not something hypothetical, doubtful, or unknown in the sense that would trigger the subjunctive.

The sentence is about finding out an actual fact:

• de dónde sale la humedad

Spanish normally uses the indicative in indirect questions like this when referring to something real:

• No sé dónde vive.
• Quiero ver qué pasa.
• Necesitan averiguar de dónde viene el ruido.

So sale is the normal choice.

Yes. A few alternatives are possible, with slightly different shades of meaning:

• La propietaria quiere saber de dónde sale la humedad.
  Slightly more neutral: wants to know.

• La propietaria quiere descubrir de dónde sale la humedad.
  More like wants to discover.

• La propietaria quiere averiguar de dónde viene la humedad.
  Also natural: where the damp comes from.

• La propietaria quiere averiguar cuál es el origen de la humedad.
  More formal: what the source of the damp is.

The original sentence sounds natural and idiomatic.

Yes, but the original order is the most neutral and natural.

Standard order:

• La propietaria quiere averiguar de dónde sale la humedad.

You might also hear emphasis in speech, but Spanish usually keeps this order in a normal statement.

The important part is that de dónde sale la humedad functions as an indirect question after quiere averiguar. That whole part works like what she wants to find out.