Vive en el sexto piso.

Vive is the third-person singular form of vivir in the present tense.

• vivo = I live
• vives = you live (informal singular)
• vive = he/she lives or you live (formal usted)

So the sentence is talking about he, she, or usted.

Spanish often leaves out subject pronouns because the verb ending already shows the person.

So instead of saying:

• Él vive en el sexto piso

Spanish very often just says:

• Vive en el sexto piso

This is completely normal. You add él, ella, or usted only if you want emphasis, contrast, or extra clarity.

Because Spanish uses en for location in many cases where English uses in or on.

So:

• en el sexto piso = literally in the sixth floor
• but in natural English we say on the sixth floor

This is just a difference in how the two languages express location. After vivir, en is the normal preposition for where someone lives.

El is the definite article meaning the.

Spanish usually uses the article when talking about a specific floor:

• el sexto piso = the sixth floor

The structure is:

• el
  • sexto
    • piso

That is the normal way to say it.

Because sexto is an ordinal number (sixth), while seis is a cardinal number (six).

• seis = six
• sexto = sixth

When talking about floors in order, Spanish normally uses the ordinal:

• el sexto piso = the sixth floor

Because sexto has to agree with piso, and piso is a masculine noun.

• el piso → masculine
• so: el sexto piso

If the noun were feminine, the ordinal would change too:

• la planta → feminine
• la sexta planta

Here it means floor/storey of a building, because it is used with an ordinal number: sexto.

However, in Spain, piso very often also means flat/apartment. Because of that, context matters.

For a building level, many speakers in Spain very naturally say:

• la sexta planta

So Vive en el sexto piso is understandable, but Vive en la sexta planta may sound more typically Spain Spanish in many situations.

Yes. In Spain, that is often a very natural alternative.

• Vive en el sexto piso
• Vive en la sexta planta

Both can mean the same thing in context: someone lives on the sixth floor.
If you are focusing on Spain Spanish, planta is especially useful for talking about levels of a building.

Because vivir means to live / reside, while estar only tells you where someone is at a given moment.

• Vive en el sexto piso = that is where the person lives
• Está en el sexto piso = the person is on the sixth floor right now

So if you mean someone’s home or residence, vive is the correct choice.

Not always. In Spain, buildings often have a planta baja or bajo at street level, and then the numbered floors start above that.

So:

• planta baja / bajo = ground floor
• primer piso / primera planta = first floor above ground level

This matches British usage more closely than American usage. So for a US English speaker, Spanish floor numbering can feel one floor higher than expected.