Metto il biberon sul seggiolone.

In Italian, subject pronouns are often omitted because the verb ending already tells you who is doing the action.

• metto = I put / I am putting
• The -o ending shows it is first person singular: I

So Metto il biberon sul seggiolone naturally means I put the baby bottle on the high chair without needing io.

You can say Io metto... if you want emphasis, for example:

• Io metto il biberon sul seggiolone, tu metti il piatto sul tavolo.
  I put the bottle on the high chair, you put the plate on the table.

Metto is the present tense of mettere.

Here is the relevant part of the conjugation:

• io metto = I put / I am putting
• tu metti = you put
• lui/lei mette = he/she puts
• noi mettiamo = we put
• voi mettete = you all put
• loro mettono = they put

In Italian, the present tense can often translate as either:

• I put
• I am putting

The exact English version depends on context.

Yes. Mettere is one of the main Italian verbs for to put, to place, or to set something somewhere.

Examples:

• Metto il libro sul tavolo. = I put the book on the table.
• Metti la chiave nella borsa. = Put the key in the bag.
• Mettiamo i piatti in cucina. = We put the plates in the kitchen.

It is a very common verb and is used in many expressions.

Italian usually uses the definite article more often than English does.

So where English might say:

• I put the baby bottle on the high chair
• or sometimes just I put baby’s bottle on the high chair depending on context

Italian normally says:

• Metto il biberon...

Here, il means the.

Even when English might omit an article in some situations, Italian often keeps it.

Biberon means baby bottle or feeding bottle.

A few useful notes:

• It is a masculine singular noun, so it takes il
• Singular: il biberon
• Plural: i biberon

It is an invariable noun in form, so the singular and plural usually look the same; the article shows the number.

Examples:

• il biberon = the baby bottle
• i biberon = the baby bottles

Because in Italian, su + il usually combines into the contracted form sul.

So:

• su = on
• il = the
• su il → sul

This is very common with prepositions and articles in Italian.

Other examples:

• su + lo = sullo
• su + la = sulla
• su + i = sui
• su + gli = sugli
• su + le = sulle

So sul seggiolone literally means on the high chair.

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

It is a masculine singular noun:

• il seggiolone = the high chair
• i seggioloni = the high chairs

So:

• sul seggiolone = on the high chair

Yes, the choice depends on meaning.

• sul seggiolone = on the high chair
• nel seggiolone = in the high chair

Use sul seggiolone if you mean the bottle is being placed on top of the high chair, for example on the tray.

Use nel seggiolone if you mean the bottle is being placed inside the high chair area, depending on the situation.

So the original sentence specifically expresses on rather than in.

Yes, Italian word order is flexible, although the basic order here is very natural:

• Metto il biberon sul seggiolone.

This is a straightforward neutral sentence.

You could also say:

• Il biberon lo metto sul seggiolone.

That version puts extra focus on il biberon, almost like:

• The baby bottle, I put it on the high chair.

So both are possible, but the original is the simplest and most neutral.

Yes. The Italian present tense often covers both:

• I put the baby bottle on the high chair
• I’m putting the baby bottle on the high chair

Context decides which English translation sounds best.

If you wanted to emphasize an action in progress, Italian can also use a progressive form:

• Sto mettendo il biberon sul seggiolone.
  = I’m putting the baby bottle on the high chair.

But in many everyday situations, simple metto is perfectly normal.

In this sentence, il seggiolone is grammatically the high chair, but Italian often uses the definite article even when English might rely more on context.

So il seggiolone may refer to:

• a specific high chair already known in the situation
• the obvious high chair in the room
• simply the relevant high chair in context

This is normal in Italian. The article does not always mean the speaker is making a strong contrast with other possible high chairs; often it just sounds natural.

Yes, it is natural and correct.

Metto il biberon sul seggiolone is a normal way to say that you are placing the baby bottle on the high chair.

Depending on context, a native speaker might also say things like:

• Metto il biberon sul vassoio del seggiolone.
  I put the bottle on the high chair tray.
• Appoggio il biberon sul seggiolone.
  I set the bottle on the high chair.

But your original sentence is perfectly normal and understandable.