In “treno” ci sono tre consonanti e due vocali.

Ci sono is the normal way to say there are in Italian.

• c’è = there is
• ci sono = there are

So:

• ci sono tre consonanti = there are three consonants
• ci sono due vocali = there are two vowels

It is an existential expression, used to say that something exists or is present.

In this sentence, it is best to learn ci sono as a whole expression meaning there are.

Historically, ci has a meaning related to there/in it, but for a learner, the most useful approach is:

• memorize c’è = there is
• memorize ci sono = there are

So here, ci is not the same as ci meaning us or to us.

Because the things being counted are plural:

• tre consonanti
• due vocali

Since the subject is plural, Italian uses the plural verb sono.

Compare:

• C’è una vocale. = There is one vowel.
• Ci sono due vocali. = There are two vowels.

Here, in means in the word treno.

So In treno means something like:

• In the word treno
• Within treno

Italian often allows this shorter structure when talking about letters, sounds, or parts of a word.

A more explicit version would be:

• Nella parola treno ci sono tre consonanti e due vocali.

That version may feel clearer to beginners.

Yes, very often in treno means by train in everyday Italian.

For example:

• Vado a Roma in treno. = I’m going to Rome by train.

But in your sentence, treno is being treated as a word, not as a means of transport. The context makes that clear, especially because the sentence is about consonants and vowels.

If you want to avoid any possible ambiguity, you can say:

• Nella parola treno...

After numbers, Italian normally uses the noun directly, without an article.

So you say:

• tre consonanti
• due vocali

not normally:

• le tre consonanti
• le due vocali

unless you mean specific consonants or vowels already identified.

Here the sentence is just giving a count, so no article is needed.

This is a very common point of confusion.

The singular forms are:

• la consonante
• la vocale

These are feminine nouns ending in -e.

In Italian, many nouns ending in -e form the plural in -i, whether they are masculine or feminine.

So:

• la consonante → le consonanti
• la vocale → le vocali

So the -i ending does not automatically mean masculine.

Because due does not change for gender.

Most Italian numbers are invariable:

• due vocali
• due consonanti
• due libri

The number stays the same whether the noun is masculine or feminine.

The main exception beginners notice is one:

• un libro
• una vocale

But due, tre, and most other numbers do not change.

It is pronounced roughly TREH-no.

A few helpful notes:

• tr is pronounced together
• the r is usually tapped or lightly rolled
• e is a clear Italian vowel, not like the changing English vowel in many words
• o is also clearly pronounced

Unlike English, Italian spelling is usually very regular, so all the letters are pronounced.

In normal Italian, ci sono is the natural way to say there are.

So:

• Ci sono tre consonanti e due vocali is the standard sentence.

If you say only:

• Sono tre consonanti e due vocali

it sounds incomplete or unnatural in this context, unless the structure is changed for emphasis.

For example, a more literary or emphatic structure could be:

• Tre sono le consonanti e due le vocali

But that is not the ordinary beginner-friendly way to say it. For normal usage, stick with ci sono.