Metto il bavaglino sul seggiolone.

Metto is the first-person singular present tense of mettere, which means to put.

So metto means:

• I put
• I am putting
• sometimes even I’ll put, depending on context

Italian present tense is often broader than English present simple.

Italian usually drops the subject pronoun when it is clear from the verb ending.

• metto = I put
• metti = you put
• mette = he/she puts

Because -o already tells you the subject is I, io is normally unnecessary. You can add io only for emphasis or contrast.

In Italian, the simple present often covers both:

• I put
• I am putting

So Metto il bavaglino sul seggiolone can naturally describe an action happening now.

If you want to strongly emphasize that the action is in progress, Italian can also use:

• Sto mettendo il bavaglino sul seggiolone

But the simple present is very common and natural.

Italian uses articles more often than English.

Here, il bavaglino and il seggiolone mean:

• the bib
• the high chair

Even when English might sometimes say a bib or use no article in a similar context, Italian often prefers the definite article when the object is understood from the situation.

Bavaglino means bib, especially a baby’s bib.

The ending -ino is a very common Italian suffix. It often suggests something small or cute, though in many words it has simply become the normal standard word. So you should learn bavaglino as the usual word for bib.

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

So:

• su + il = sul

This is very similar to other common contractions:

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

So sul seggiolone literally means on the high chair.

Su means on or onto.

In sul seggiolone, it shows that the bib is being placed on the high chair, not in it or to it.

So the structure is:

• mettere qualcosa su qualcosa = to put something on something

That is why su is the natural preposition here.

Yes, Italian word order is fairly flexible.

The most neutral order here is:

• Metto il bavaglino sul seggiolone.

But you could also say:

• Sul seggiolone metto il bavaglino.

That version puts more emphasis on where you are putting it.

So the original sentence is the most straightforward, everyday version.

Seggiolone means high chair, the chair used for feeding a baby or small child.

It is a specific vocabulary word, so it is best learned as a whole word rather than translated piece by piece every time.

You could say:

• Io metto il bavaglino sul seggiolone.

This is grammatically correct, but it sounds more emphatic, like:

• I’m the one putting the bib on the high chair
• I put it on the high chair, not someone else

Without special emphasis, the version without io is more natural.