Antes de salir, apago la plancha y cierro la puerta.

Spanish often drops subject pronouns when the verb already shows who is doing the action.

• apago = I turn off
• cierro = I close

So yo is not necessary. You could say Yo apago la plancha y cierro la puerta, but that usually adds emphasis or contrast.

For example:

• Yo apago la plancha, pero tú dejas todo encendido.
  = I turn off the iron, but you leave everything on.

After antes, Spanish normally uses de + infinitive when the same person does both actions.

So:

• Antes de salir, apago la plancha...
  literally: Before leaving, I turn off the iron...

This is the standard pattern for before doing something.

Other examples:

• Antes de comer, me lavo las manos.
• Antes de dormir, leo un poco.

Because it comes after the preposition de in the structure antes de + infinitive.

In Spanish, after many prepositions, you use the infinitive, not a form like English leaving.

So English says:

• Before leaving

Spanish says:

• Antes de salir

Not antes de salgo and not antes de saliendo.

Use antes de que + subjunctive when the subject changes.

In your sentence, the same person does everything, so antes de salir is correct.

Compare:

• Antes de salir, apago la plancha.
  The same person leaves and turns off the iron.

But:

• Antes de que salgas, apago la plancha.
  You leave, but I turn off the iron.

So:

• same subject → antes de + infinitive
• different subject → antes de que + subjunctive

Spanish often uses the present tense to describe a habit, routine, or something that generally happens.

So this sentence sounds like:

• Before leaving, I turn off the iron and close the door.

That is, this is what I do.

If you wanted a past action, you would use the preterite:

• Antes de salir, apagué la plancha y cerré la puerta.
  = Before leaving, I turned off the iron and closed the door.

Because cerrar is a stem-changing verb in the present tense.

The e changes to ie in most forms:

• yo cierro
• tú cierras
• él/ella cierra
• nosotros cerramos
• vosotros cerráis
• ellos cierran

So cierro la puerta is the correct yo form.

This kind of change is very common in Spanish.

Here, la plancha means the iron — the household appliance used for ironing clothes.

In context, apagar la plancha very naturally means to turn off the iron.

The noun plancha can have other meanings in other contexts, but in this sentence the intended meaning is clearly the appliance.

Because the verbs match the type of object:

• apagar = to turn off / switch off
  used for things with power, light, heat, etc.
• cerrar = to close / shut
  used for doors, windows, shops, books, etc.

So:

• apagar la plancha = turn off the iron
• cerrar la puerta = close the door

You would not normally say cerrar la plancha in this meaning.

Because Antes de salir is an introductory phrase, and a comma helps separate it from the main clause.

• Antes de salir, apago la plancha y cierro la puerta.

This is very natural and clear.

In Spanish, short introductory phrases are sometimes written without a comma, especially in less formal writing, but the comma here is perfectly correct and often preferred.

Because Spanish usually uses an article with a specific noun like this.

So:

• cierro la puerta = I close the door

It sounds natural because it refers to the relevant door in the situation, usually the one you use when leaving.

English sometimes behaves similarly, but Spanish is often a bit more consistent about using articles in these cases.

In most of Spain, cierro is pronounced roughly like THYEH-rro:

• the ci sounds like the th in thin
• the e is a clear short vowel
• the rr is rolled or strongly tapped

A rough guide is:

• cierro → THYEH-rro

In many Latin American varieties, it would sound more like SYEH-rro.

Since you asked about Spanish from Spain, the th-like sound is the usual reference point.