È decisamente troppo costoso.

Because è is the verb essere in the he/she/it is or is form. The accent is important because e without an accent usually means and.

So:

• è = is
• e = and

This is a very common spelling point in Italian.

Italian often leaves out subject pronouns when they are already understood from the context or from the verb form. This is called being a pro-drop language.

So instead of saying the equivalent of It is definitely too expensive, Italian can simply say:

• È decisamente troppo costoso.

The it is understood from the situation.

Decisamente is an adverb. Here it strengthens the statement, giving the idea of definitely, clearly, or decidedly.

It modifies the whole idea that something is too expensive.

So the structure is roughly:

• È = is
• decisamente = definitely / decidedly
• troppo costoso = too expensive

Because troppo is modifying the adjective costoso.

In Italian, adverbs such as troppo, molto, abbastanza, and poco usually come before the adjective they modify:

• troppo costoso = too expensive
• molto costoso = very expensive
• abbastanza costoso = fairly expensive

So troppo comes before costoso for the same reason too comes before expensive in English.

The adjective must agree with the noun it refers to. Costoso is the masculine singular form.

Possible forms are:

• costoso = masculine singular
• costosa = feminine singular
• costosi = masculine plural
• costose = feminine plural

So if the thing being talked about is masculine singular, costoso is correct. For example:

• Il computer è decisamente troppo costoso.
• La macchina è decisamente troppo costosa.

They are similar, but not always identical.

• costoso usually emphasizes that something has a high cost or is expensive in price
• caro often means expensive too, but it is more common in everyday speech and can also mean dear in other contexts

For example:

• Questo ristorante è costoso. = This restaurant is expensive.
• Questo ristorante è caro. = This restaurant is expensive.

Both can work, but caro is often more conversational. Costoso can sound slightly more formal or descriptive depending on context.

They do different jobs.

• troppo means too, so it says the price goes beyond an acceptable limit
• decisamente adds emphasis, like definitely or clearly

So:

• È troppo costoso. = It’s too expensive.
• È decisamente troppo costoso. = It’s definitely / clearly too expensive.

Using both makes the judgment stronger.

Yes, but it means something different.

• È molto costoso = It is very expensive.
• È troppo costoso = It is too expensive.

Molto costoso just describes a high price. Troppo costoso suggests that the price is excessive or unacceptable in the situation.

So troppo is stronger in a more practical way.

They can move somewhat depending on style and emphasis, but in a sentence like this, putting decisamente before troppo costoso is very natural:

• È decisamente troppo costoso.

You may also hear similar patterns such as:

• È davvero troppo costoso.
• È veramente troppo costoso.
• È assolutamente troppo costoso.

For learners, a good safe pattern is:

essere + adverb + troppo + adjective

A simple pronunciation guide is:

eh deh-chee-zah-men-teh TROHP-poh coh-STOH-zoh

A few helpful points:

• È sounds like eh
• ci in decisamente sounds like chee
• troppo has a doubled pp, so the consonant is held a little longer
• stress falls on -so- in costoso

If you want to sound natural, give a little stress to decisamente and troppo.

Yes. It sounds natural, especially when the thing being discussed is already clear from context.

For example:

• Vuoi comprarlo?
• No, è decisamente troppo costoso.

Because Italian often leaves out subjects, a sentence like this works perfectly well by itself when everyone knows what it refers to.