Tutti sentono la sirena.

Here tutti means everyone or all people.

Literally, it is the masculine plural form of tutto (all / whole / every), but in a sentence like this it functions like a pronoun: everyone.

A useful detail:

• tutti = all / everyone, for a mixed group or a group of males
• tutte = all / everyone, for a group of females

So if you were talking only about women, you could say Tutte sentono la sirena.

Because tutti is grammatically plural.

The verb comes from sentire (to hear, and sometimes to feel), and sentono is the third person plural form in the present tense:

• io sento = I hear
• tu senti = you hear
• lui/lei sente = he/she hears
• noi sentiamo = we hear
• voi sentite = you all hear
• loro sentono = they hear

Since tutti means all people / everyone, Italian uses the plural verb: sentono.

Italian often leaves out subject pronouns when they are not needed.

So instead of saying:

• Loro sentono la sirena

Italian very naturally says:

• Tutti sentono la sirena

or even just:

• Sentono la sirena

if the subject is clear from context.

This is because the verb ending already tells you the person and number. The ending -ono shows that the verb is third person plural.

It can mean both, depending on context.

In this sentence, sentire means to hear:

• Tutti sentono la sirena = everyone hears the siren

But in other contexts it can mean to feel:

• Sento freddo = I feel cold
• Ti senti bene? = Do you feel well?

So sentire is a flexible verb, and context tells you which meaning is intended.

Because sirena is a feminine singular noun, so it takes the feminine singular definite article la.

• il = masculine singular
• la = feminine singular
• i / gli = masculine plural
• le = feminine plural

So:

• la sirena = the siren

In Italian, articles are used very regularly, often more regularly than in English. Here, la sirena simply means the siren.

Yes. Sirena can mean both:

• siren / alarm siren
• mermaid
• sometimes even mythological siren

The context tells you which meaning is intended.

In Tutti sentono la sirena, the meaning is clearly siren as a sound-producing alarm, because people hear it.

In Italian, the masculine plural is the default form for a mixed group or for people in general.

So tutti can mean:

• all the men
• all the people in a mixed group
• everyone in a general sense

If the group is entirely female, Italian uses tutte.

This is very common in Italian grammar:

• Tutti sono pronti. = Everyone is ready.
• Tutte sono pronte. = All of them are ready. (all female)

Yes. Tutti sentono la sirena is the neutral, straightforward order: subject + verb + object.

But Italian can change word order for emphasis. For example:

• La sirena la sentono tutti.

This puts emphasis on the siren or on the fact that everyone hears it.

The original sentence is the most basic and natural way to say it in an ordinary context.

A rough pronunciation guide is:

TOOT-tee SEN-toh-noh lah see-REH-nah

A few points:

• tutti has a doubled tt, so the t sound is held a little longer than in English
• sentono has the stress on sen
• sirena has the stress on re: si-RE-na

If you want to sound more natural, try to keep the vowels clear and short:

• tu
• tti
• sen
• to
• no
• la
• si
• re
• na

Yes. Tutti can stand alone, as it does here, or it can go with a noun.

For example:

• Tutti sentono la sirena. = Everyone hears the siren.
• Tutti i vicini sentono la sirena. = All the neighbors hear the siren.
• Tutte le persone sentono la sirena. = All the people hear the siren.

When a noun follows, tutti agrees with that noun:

• tutti i ragazzi
• tutte le ragazze

So in your sentence, tutti is being used on its own, meaning everyone.