Cuando tenga tiempo, prepararé crema de calabaza y una ensalada de aguacate para cenar.

Because this sentence refers to a future situation: when I have time.

In Spanish, after cuando, if the action is in the future, you normally use the present subjunctive:

• Cuando tenga tiempo, ... = When I have time, ...

If you say cuando tengo tiempo, that usually means a habitual or general situation:

• Cuando tengo tiempo, cocino. = When I have time, I cook.

So in your sentence, tenga is used because the speaker is talking about a future moment that has not happened yet.

Tenga is the present subjunctive form of tener.

The verb tener is irregular, and its present subjunctive forms are:

• yo tenga
• tú tengas
• él/ella/usted tenga
• nosotros/nosotras tengamos
• vosotros/vosotras tengáis
• ellos/ellas/ustedes tengan

Here, tenga means I have, but in the subjunctive mood.

Because the speaker is saying what they will do in the future:

• prepararé = I will prepare / I’ll make

The structure is:

• Cuando tenga tiempo = when I have time
• prepararé... = I will prepare...

So the whole sentence describes a future plan or intention.

Yes. In everyday Spanish, voy a preparar is very common and natural.

So you could say:

• Cuando tenga tiempo, voy a preparar crema de calabaza y una ensalada de aguacate para cenar.

Both versions are correct:

• prepararé = simple future
• voy a preparar = going to prepare

In conversation, the ir a + infinitive form is often especially common.

Because Spanish usually does not need subject pronouns when the verb ending already shows who the subject is.

• prepararé already means I will prepare

So yo is optional:

• Cuando tenga tiempo, prepararé...
• Cuando tenga tiempo, yo prepararé...

Adding yo can give emphasis or contrast, but it is not necessary.

Because tener tiempo is a fixed, very common expression meaning to have time.

So Spanish normally says:

• Tengo tiempo
• No tengo tiempo
• Cuando tenga tiempo

Using el tiempo here would sound unnatural in this meaning.

Be careful: el tiempo often means time in a more general sense, or weather, depending on context.

In Spain, crema de calabaza usually means a creamy pumpkin soup or pumpkin purée-style soup.

A crema is often smoother and creamier than a more general sopa. In many contexts, English might simply translate it as pumpkin soup.

So yes, it is basically a kind of soup, but crema suggests a smooth, creamy texture.

Because de is commonly used in Spanish to show the main ingredient or what something is made with.

• crema de calabaza = pumpkin cream / pumpkin soup
• ensalada de aguacate = avocado salad

This is very common with foods:

• zumo de naranja = orange juice
• tarta de queso = cheesecake
• ensalada de tomate = tomato salad

Because para expresses purpose here:

• para cenar = for dinner / to have for dinner

The idea is: the speaker will prepare those dishes in order to eat them at dinner.

Por would not be the normal choice in this sentence.

Yes. It can be understood in both ways, depending on how you translate naturally into English.

• Prepararé ... para cenar can mean:
  • I’ll make ... for dinner
  • I’ll make ... to have for dinner

Spanish often uses para + infinitive to show purpose, and in English there may be more than one natural way to express that.

Because the sentence begins with a subordinate clause:

• Cuando tenga tiempo, ...

In English, we also usually put a comma after an opening clause:

• When I have time, I’ll make...

In Spanish, this comma is natural and helpful for clarity when the cuando clause comes first.

Yes. You can say:

• Prepararé crema de calabaza y una ensalada de aguacate para cenar cuando tenga tiempo.

That is grammatically correct.

However, the version with Cuando tenga tiempo at the beginning may sound slightly more natural if you want to emphasize the condition/time frame first.

Because after para, Spanish often uses an infinitive when the subject is the same or when the action is being expressed generally.

• para cenar = to have for dinner / for dinner

This is very common:

• para comer = to eat / for lunch or food
• para beber = to drink
• para llevar = to take away / takeaway

So cenar stays in the infinitive form.

Yes, in many contexts you could say:

• ... para la cena

That would mean for dinner as a noun phrase.

There is a slight difference:

• para cenar focuses more on the action of eating dinner
• para la cena focuses more on the meal itself

In everyday use, both can work, but para cenar sounds very natural in this sentence.

Yes. In Spain, aguacate is the normal word for avocado.

So:

• una ensalada de aguacate = an avocado salad

That is standard and natural Spanish.