La propietaria del piso es amable.

Because propietaria is the feminine form of propietario.

• el propietario = the male owner / landlord
• la propietaria = the female owner / landlady

The article also changes to match:

• la
  • feminine noun
• el
  • masculine noun

So La propietaria del piso tells you the owner is a woman.

Propietaria means owner. In this context, it refers to the woman who owns the flat/apartment.

Depending on context, in English you might translate it as:

• owner
• landlady
• female landlord

It is a fairly neutral, standard word.

Del is a contraction of de + el.

• de = of / from
• el = the
• del = of the

So:

• la propietaria del piso = the owner of the flat

This contraction is mandatory in Spanish when de is followed by el.

Compare:

• de + el = del
• de + la = de la (no contraction)

Because piso is a masculine noun:

• el piso = the flat / apartment

Since it is masculine singular, de + el becomes del.

So:

• del piso = of the flat

If the noun were feminine, there would be no contraction:

• de la casa = of the house

In Spain, piso commonly means flat or apartment.

This is very common in Peninsular Spanish. A learner may know piso as floor as well, and that is also possible depending on context, but here del piso clearly means of the flat/apartment.

In Spain:

• un piso = a flat/apartment

In other varieties of Spanish, apartamento or departamento may be more common.

Because amable is being used as a general characteristic of the person, not a temporary state.

• ser is used for identity, description, and more lasting qualities
• estar is used for states, conditions, and locations

So:

• La propietaria del piso es amable = The landlady/owner is kind.
  This sounds like a description of what she is like.

Using está amable would sound unusual here and could suggest she is being kind at the moment, rather than describing her personality.

Because amable is an adjective that has the same form for masculine and feminine.

So you can say:

• el propietario es amable
• la propietaria es amable

The adjective only changes for number:

• amable = singular
• amables = plural

Examples:

• La propietaria es amable.
• Los propietarios son amables.

Not always exactly. Amable usually means kind, friendly, or pleasant.

In many contexts, English might translate it as nice, but amable often focuses more on someone being courteous or kind in their manner.

So in this sentence, it suggests the owner is pleasant and kind to deal with.

In this sentence, la is needed because you are talking about a specific person:

• La propietaria del piso = the owner of the flat

Without the article, propietaria would not work naturally here as the subject of the sentence.

Spanish uses definite articles very regularly with nouns, often more than English does.

The structure is:

Subject + verb + adjective

• La propietaria del piso = subject
• es = verb
• amable = adjective

More specifically:

• La propietaria = the owner
• del piso = of the flat
• es amable = is kind

So the sentence literally breaks down as: The owner of the flat is kind.

Yes, you could.

• dueña = owner / landlady
• propietaria = owner / female proprietor

Both can work, but there is a slight nuance:

• propietaria sounds a bit more formal or precise
• dueña is often more everyday and direct

So La dueña del piso es amable is also natural.

It is pronounced roughly like:

pro-pye-ta-RYA

The stress falls on ta in standard Spanish spelling terms, because the word ends in a vowel and follows normal stress rules: pro-pie-TA-ria.

A rough full pronunciation of the sentence is:

La pro-pie-TA-ria del PEE-so es a-MA-ble.

Yes. Propietaria simply means female owner.

It does not automatically mean someone who rents property out. It just means she owns something. In this sentence, because it says del piso, we understand she is the female owner of the flat, and in context that may be the landlady.

So the exact English translation depends on context:

• the owner of the flat
• the landlady