Cambio il pannolino sul fasciatoio.

Cambio is the first-person singular present indicative of cambiare.

So it means:

• I change
• I am changing (depending on context)

Examples:

• Cambio il pannolino ogni giorno. = I change the diaper every day.
• Adesso cambio il pannolino. = I’m changing the diaper now / I’ll change the diaper now.

Italian often leaves out subject pronouns when they are already clear from the verb form.

• Cambio already tells you the subject is I
• So io is usually unnecessary

You could say Io cambio il pannolino sul fasciatoio, but that usually adds emphasis, for example:

• Io cambio il pannolino, tu prepara il biberon.
  = I’ll change the diaper, you prepare the bottle.

In Italian, the present tense often covers both:

• I change
• I am changing

So Cambio il pannolino can mean either, depending on context.

If you want to strongly emphasize that the action is happening right now, Italian can also use:

• Sto cambiando il pannolino.

But in everyday speech, the simple present is very common and natural.

Italian usually uses an article where English often does too, and sometimes even where English leaves it out.

Here, il pannolino means the diaper. This is natural because it refers to the specific diaper being changed.

Compare:

• Cambio il pannolino. = I’m changing the diaper.
• Cambio un pannolino. = I’m changing a diaper / one diaper.

Using un pannolino would sound less specific and is not the most natural choice in this everyday context.

Yes. Pannolino is a masculine singular noun, so it takes il.

• singular: il pannolino
• plural: i pannolini

This is something you simply have to learn with the noun.

Here cambiare means to change / replace the diaper.

Italian uses cambiare very naturally for this idea:

• cambiare il pannolino = to change the diaper
• cambiare i vestiti = to change clothes
• cambiare le lenzuola = to change the sheets

So this is a very normal verb choice.

Because in Italian, su + il normally contracts to sul.

So:

• su + il = sul

This is just like:

• a + il = al
• di + il = del
• in + il = nel

Since fasciatoio is masculine singular and takes il, su il fasciatoio becomes sul fasciatoio.

It is a prepositional phrase telling you where the action happens.

• su = on
• il fasciatoio = the changing table
• sul fasciatoio = on the changing table

So the sentence structure is:

• Cambio = verb
• il pannolino = direct object
• sul fasciatoio = place/location

Literally, it means on the changing table, and that is the basic meaning here.

In real use, it naturally gives the idea that the diaper-changing is happening there. So in English, depending on context, you might understand it as:

• on the changing table
• or more loosely at the changing table

But the Italian preposition su is still the normal choice because a changing table is thought of as a surface.

This is the most neutral Italian word order:

verb + object + place

So:

• Cambio = I change / I’m changing
• il pannolino = the diaper
• sul fasciatoio = on the changing table

Italian can move things around for emphasis, though:

• Sul fasciatoio cambio il pannolino.
  This puts more focus on the location.

But the original sentence is the most straightforward and natural.

Yes. That is also correct.

The difference is mainly one of aspect:

• Cambio il pannolino sul fasciatoio = neutral, very natural, can mean I change / I’m changing the diaper on the changing table
• Sto cambiando il pannolino sul fasciatoio = more explicitly I am in the middle of changing the diaper on the changing table

Both are good. The version with cambio is often simpler and more idiomatic in everyday Italian.

Yes. You can replace it with lo, because pannolino is masculine singular.

So:

• Cambio il pannolino sul fasciatoio.
• Lo cambio sul fasciatoio. = I change it / I’m changing it on the changing table.

Notice that the pronoun usually goes before the verb in Italian.

Fasciatoio is the normal Italian word for a changing table or changing station for a baby.

It is a masculine singular noun:

• il fasciatoio
• i fasciatoi

It is a common everyday word in contexts involving babies and childcare.

Usually sul fasciatoio is better here.

Why?

• sul focuses on the baby being placed on the surface of the changing table
• al would sound more like at/to the changing table, which is less precise for this action

So for actually changing a diaper on that piece of furniture, sul fasciatoio is the most natural choice.