Quando la zucca è cotta al forno, mia sorella la mangia volentieri.

Here quando can be understood as when or whenever, depending on context.

In this sentence, because the verbs are in the present tense and the statement sounds habitual, it often has the sense of whenever:

Quando la zucca è cotta al forno, mia sorella la mangia volentieri.
= Whenever pumpkin is baked, my sister gladly eats it.

So quando introduces a time clause, and in sentences like this it often expresses a repeated situation.

After quando, Italian normally uses the indicative when the speaker is referring to something real, factual, or habitual.

So quando la zucca è cotta al forno means when/whenever the pumpkin is cooked in the oven.

The subjunctive is not needed here, because nothing is being presented as doubtful, emotional, or hypothetical.

If you were talking about a specific future moment, you might hear something like:

Quando sarà cotta al forno...
= When it has been baked / when it is baked

But in your sentence, the present indicative works because the meaning is general and habitual.

Cotta is the past participle of cuocere, which means to cook.

So:

• cuocere = to cook
• cotto / cotta = cooked

In è cotta, the structure is:

• è = is
• cotta = cooked

So literally it means is cooked.

In context, è cotta al forno means is cooked in the oven / is oven-baked / is baked.

Because zucca is a feminine singular noun.

In Italian, past participles used with essere often agree with the noun they describe:

• masculine singular: cotto
• feminine singular: cotta
• masculine plural: cotti
• feminine plural: cotte

Since la zucca is feminine singular, the correct form is cotta.

Al forno is a very common Italian food expression meaning baked, oven-baked, or cooked in the oven.

Literally, it comes from a + il forno, which becomes al forno.

You will see it in many food names, for example:

• patate al forno = baked potatoes
• pasta al forno = baked pasta

So zucca cotta al forno means pumpkin cooked by baking it in the oven.

The first la in la zucca is the definite article: the.

The second la in la mangia is a direct object pronoun meaning it.

So the sentence first introduces the thing:

la zucca = the pumpkin / pumpkin

Then, in the main clause, Italian refers back to it with la:

mia sorella la mangia = my sister eats it

This avoids repeating la zucca again. English does the same thing:

When the pumpkin is baked, my sister eats it willingly.

In Italian, singular possessives with close family members usually do not take the definite article.

So you say:

• mia sorella = my sister
• mio fratello = my brother
• mia madre = my mother

This is normal Italian.

But with plurals, you usually do use the article:

• le mie sorelle = my sisters

And with some modified family words, the article often returns:

• la mia sorellina = my little sister

Volentieri means gladly, willingly, or with pleasure.

So:

la mangia volentieri
means
she gladly eats it / she likes eating it

It is a very common Italian word when talking about doing something happily or willingly:

• Vengo volentieri. = I’ll gladly come.
• Lo faccio volentieri. = I’m happy to do it.

In this sentence, it shows that the sister enjoys eating pumpkin in that form.

Because Italian often uses the present tense for general truths, habits, and repeated actions.

This sentence does not sound like a one-time event. It sounds like a usual pattern:

• when the pumpkin is baked,
• my sister eats it gladly.

So the present tense expresses a habitual fact, just as English can do:

When pumpkin is baked, my sister gladly eats it.

Yes. You could also say:

Mia sorella la mangia volentieri quando la zucca è cotta al forno.

That is still correct.

Putting the quando clause first gives it a slightly more natural setup in this sentence, because it establishes the situation before telling us what the sister does.

Also, when the subordinate clause comes first, Italian commonly uses a comma:

Quando la zucca è cotta al forno, ...

Italian uses definite articles more often than English does.

So la zucca can refer to:

• the pumpkin in a specific context, or
• pumpkin as a food item being talked about generally

English often drops the article more easily, but Italian usually prefers the article with nouns like this.

That is why la zucca sounds natural here, even if in English you might simply say pumpkin.

You could, but cotta al forno is more natural here.

• cotta al forno = cooked/baked in the oven
• cucinata al forno = cooked in the oven

Cucinata is understandable, but it sounds less idiomatic and a bit more general.
For food, cotto/a al forno is the more usual way to describe something that has been baked.

So zucca cotta al forno is the better choice.