¿Por qué andas tocando el pomo de esa puerta si no es tu piso?

Andar + gerund is a common colloquial structure in Spanish. It often suggests that someone is going around doing something, and it can carry a nuance of annoyance, suspicion, or criticism.

So:

• ¿Por qué estás tocando el pomo...? = more neutral, simply Why are you touching the doorknob...?
• ¿Por qué andas tocando el pomo...? = more like Why are you going around messing with the doorknob...?

In this sentence, andas tocando makes the speaker sound more reproachful.

Andas is the 2nd person singular of andar in the present tense, so it means you walk / you go around / you are... depending on context.

Spanish often drops subject pronouns when they are not needed, because the verb form already shows who the subject is.

So:

• ¿Por qué andas tocando...? = normal
• ¿Por qué tú andas tocando...? = possible, but more emphatic or contrastive

Adding tú would usually sound like the speaker is stressing you in particular.

Because this is a direct question.

Spanish distinguishes:

• por qué = why
• porque = because
• el porqué = the reason
• por que = a less common combination used in specific structures

Here the sentence asks why, so it must be ¿Por qué...?

Because tocar takes a direct object when it means to touch.

So:

• tocar el pomo = to touch the doorknob
• tocar la puerta = to touch the door

There is no personal a here because el pomo is an object, not a person. The personal a is mainly used with specific human direct objects.

In Spain, pomo usually means a doorknob or sometimes the knob-like part of a door handle.

Depending on the door, English might translate it as:

• doorknob
• door knob
• sometimes handle, if the exact shape is not important

This is a very Spain-friendly word. In other Spanish-speaking countries, other words may be more common, such as manija, perilla, or picaporte, depending on the region and the exact type of handle.

Spanish demonstratives show relative distance or perspective:

• esta = this (near the speaker)
• esa = that (near the listener, or not especially near the speaker)
• aquella = that over there (far from both, or felt as more distant)

So esa puerta means that door. It suggests the speaker is referring to a specific door that is not presented as this one right here by me.

In real speech, esa is often the natural choice for referring to something the other person is dealing with.

In Spain, piso very often means flat or apartment.

So here tu piso means your flat / your apartment, not your floor.

It is true that piso can also mean floor in other contexts, but in Peninsular Spanish it is extremely common to use piso for a dwelling.

That is why this sentence sounds very natural in Spain.

Both are possible, but they are not exactly the same in effect.

• no es tu piso = it isn’t your flat
• no es tuyo = it isn’t yours

Using tu piso is more explicit and fits the situation clearly: the speaker is saying that the apartment behind that door does not belong to the person.

Also note that with a possessive adjective like tu, Spanish normally does not use an article:

• tu piso
• not el tu piso

Because the speaker is treating this as a real fact, not a hypothetical possibility.

With ordinary si clauses about real or assumed-real situations, Spanish uses the indicative:

• si no es tu piso... = if it isn’t your flat...

Spanish does not use the subjunctive after si in this kind of present condition.

If the sentence were hypothetical, it would look different:

• si no fuera tu piso... = if it weren’t your flat...

But that is not the structure here.

Literally, it means touching / handling the doorknob. But in context, it very likely suggests more than a casual touch.

Depending on the situation, it can imply:

• grabbing the knob
• turning it
• testing the door
• fiddling with it
• possibly trying to get in

So even though the verb is just tocar, the overall sentence makes the action sound suspicious.

It sounds accusatory, suspicious, and reproachful.

Several things create that tone:

• ¿Por qué...? = direct challenge
• andas + gerund = suggests improper or irritating ongoing behavior
• si no es tu piso = reminds the person that they have no business there

So this is not a neutral question. It sounds more like:

Why are you messing with that door handle when it’s not even your flat?

The speaker is clearly confronting the other person.