I broccoli cuociono nella pentola.

Because broccoli is plural.

In Italian:

• il broccolo = the broccoli plant / one broccoli
• i broccoli = the broccoli / the broccolis / broccoli pieces in general

The article has to match the noun in gender and number:

• il = masculine singular
• i = masculine plural

So i broccoli is correct because broccoli is masculine plural.

Italian has two common masculine plural definite articles:

• i
• gli

You use i before most consonants:

• i broccoli
• i libri
• i piatti

You use gli before:

• vowels: gli amici
• z: gli zaini
• s
  • consonant: gli studenti
• gn, ps, x, and similar sounds: gli gnocchi, gli psicologi

Since broccoli starts with b, the correct article is i.

Because the subject i broccoli is third person plural, so the verb must also be plural.

The verb is cuocere = to cook.

Present tense:

• io cuocio
• tu cuoci
• lui/lei cuoce
• noi cuociamo
• voi cuocete
• loro cuociono

So:

• il broccolo cuoce = the broccoli cooks / is cooking
• i broccoli cuociono = the broccoli cook / are cooking

This is simple subject-verb agreement.

Because cuocere is not fully regular in the present tense.

Many Italian verbs have small spelling or stem changes in certain forms. With cuocere, the -ci- appears in some forms:

• cuocio
• cuoci
• cuoce
• cuociamo
• cuocete
• cuociono

So cuociono is just the correct present-tense form you need to learn.

It can mean either, depending on context.

Italian simple present often covers both:

• they cook
• they are cooking

So I broccoli cuociono nella pentola could mean:

• The broccoli cook in the pot
• The broccoli are cooking in the pot

Italian often uses the simple present where English prefers the progressive.

Because nella is a contraction of:

• in + la = nella

Since pentola is feminine singular, it takes la:

• la pentola = the pot

So:

• in la pentola → nella pentola

This kind of contraction is very common in Italian.

Examples:

• nel piatto = in the plate
• nella cucina = in the kitchen
• nei bicchieri = in the glasses

Italian uses articles more often than English.

So where English might say:

• in the pot
• sometimes even just in pot in very special contexts

Italian normally says:

• nella pentola

Without the article, in pentola can exist in some set expressions or special contexts, but for a basic neutral sentence, nella pentola is the normal choice.

The noun pentola is simply a feminine singular noun.

A lot of Italian nouns ending in -a are feminine:

• la pentola
• la tavola
• la cucina

So here:

• la pentola = the pot
• nella pentola = in the pot

Like many vocabulary items, the gender has to be learned with the noun.

Yes, you could.

Italian word order is more flexible than English. All of these can be natural depending on emphasis:

• I broccoli cuociono nella pentola.
• Nella pentola cuociono i broccoli.

The original sentence is straightforward and neutral.
Starting with nella pentola puts a little more focus on where the cooking is happening.

It can, but it does not have to.

Italian has a progressive form:

• I broccoli stanno cuocendo nella pentola = The broccoli are cooking in the pot

But Italian uses the simple present much more often than English does. So:

• I broccoli cuociono nella pentola
• I broccoli stanno cuocendo nella pentola

Both can work, but the simple present is very normal.

Yes. In Italian, broccoli is the plural form of broccolo.

So grammatically:

• broccolo = singular
• broccoli = plural

This is one place where Italian and English do not behave exactly the same way. English often treats broccoli as a mass noun, but Italian clearly treats broccoli as a plural noun in forms like this sentence.

Yes, but it means something different.

• I broccoli cuociono nella pentola = The broccoli are cooking in the pot
• I broccoli sono nella pentola = The broccoli are in the pot

So cuociono adds the idea of the cooking process, not just location.

The dictionary form is cuocere, meaning to cook.

When you look up verbs in Italian dictionaries, you normally look for the infinitive form, such as:

• cuocere = to cook
• mangiare = to eat
• aprire = to open

So if you see cuociono, the verb you want to recognize is cuocere.