Quand il fait chaud, ma fille préfère une tranche d’ananas, alors que mon fils choisit une mangue.

In French, weather is often expressed with faire rather than être.

• il fait chaud = it’s hot / the weather is hot
• literally, it is more like it makes heat, but you should learn it as a fixed weather expression

Other common examples:

• il fait froid = it’s cold
• il fait beau = the weather is nice
• il fait mauvais = the weather is bad

The il here does not refer to a real person or thing. It is just a grammatical subject, like it in it’s raining.

Because il fait chaud is the normal idiomatic way to talk about the weather being hot.

• quand il fait chaud = when the weather is hot / when it’s hot
• quand c’est chaud usually means when it is hot in the sense of a specific thing being hot, such as food, water, or an object

So:

• Quand il fait chaud, ... = when the weather is hot
• Quand la soupe est chaude, ... = when the soup is hot

Yes, lorsque would also be correct:

• Lorsque il fait chaud... is normally written Lorsqu’il fait chaud...

But quand is more common and more natural in everyday speech.
Lorsque often sounds a little more formal or literary.

So in ordinary conversation, quand is usually the best choice.

These are possessive adjectives:

• ma = my, used before a feminine singular noun
• mon = my, used before a masculine singular noun

So:

• fille is feminine → ma fille
• fils is masculine → mon fils

Compare:

• mon frère = my brother
• ma sœur = my sister

A useful extra point: before a feminine noun starting with a vowel sound, French often uses mon instead of ma for pronunciation reasons:

• mon amie = my female friend

But that does not apply here, because fille does not begin with a vowel sound.

Préfère is the third-person singular form of the verb préférer in the present tense.

Subject: ma fille = she
So the verb must be the she/he form:

• je préfère
• tu préfères
• il/elle préfère

The accent pattern changes in some forms of préférer. This is a common stem change:

• infinitive: préférer
• present singular forms: je préfère, tu préfères, il/elle préfère

So the sentence uses préfère because the subject is ma fille.

Because une tranche d’ananas means a slice of pineapple, not a pineapple.

French is being specific about the form of the fruit:

• un ananas = a whole pineapple
• une tranche d’ananas = a slice of pineapple

This is very common with food:

• une tranche de pain = a slice of bread
• un morceau de fromage = a piece of cheese
• un verre d’eau = a glass of water

So tranche tells you that the daughter prefers only one slice, not the whole fruit.

Because de contracts before a vowel sound.

• de + ananas becomes d’ananas

This is called elision. French often drops the final vowel of a short word before a following vowel:

• de ananas → d’ananas
• le ananas would become l’ananas
• je aime becomes j’aime

So une tranche d’ananas is simply the normal, required form.

After an expression of quantity or portion, French usually uses de without a definite or indefinite article.

So:

• une tranche d’ananas = a slice of pineapple
• un morceau de gâteau = a piece of cake
• un verre d’eau = a glass of water

You would say de l’ananas only in a different structure, for example when talking about some pineapple in general:

• Je mange de l’ananas. = I’m eating pineapple.

But after une tranche, the normal pattern is:

• une tranche de + noun
• if the noun begins with a vowel: une tranche d’ananas

Here, alors que introduces a contrast. It means something like:

• whereas
• while
• but

So the sentence is contrasting the daughter’s preference with the son’s choice.

Examples:

• Ma fille préfère une tranche d’ananas, alors que mon fils choisit une mangue.
• My daughter prefers a slice of pineapple, whereas my son chooses a mango.

It does not mean time here. It is not while in the sense of two actions happening at the same time; it is more about opposition or difference.

Yes, mais could work, but it changes the feel slightly.

• ... alors que mon fils choisit une mangue sounds like a smoother contrast between the two children
• ... mais mon fils choisit une mangue sounds simpler and more direct: but my son chooses a mango

So both are possible, but alors que is a bit more elegant and explicitly contrasts the two preferences.

Choisit is the third-person singular present form of choisir.

Subject: mon fils = he
So the verb form must be he chooses:

• je choisis
• tu choisis
• il/elle choisit

This is a regular -ir verb of the choisir type, so it follows a common pattern.

Because the sentence is stating a general habit or typical preference.

• Quand il fait chaud, ma fille préfère... = When it’s hot, my daughter prefers...
• ...mon fils choisit... = my son chooses...

French often uses the present tense for habitual actions, just as English does:

• When it’s hot, she prefers...
• My son chooses...

So this is not about one single future event. It is about what they generally do in that situation.

Because mangue is a feminine noun in French.

So you say:

• une mangue = a mango

Compare:

• une pomme = an apple
• une orange = an orange
• un ananas = a pineapple

The article has to match the gender of the noun:

• masculine singular → un
• feminine singular → une

It is standard and very natural to put a comma after an introductory clause like this:

• Quand il fait chaud, ma fille préfère...

The comma helps separate the time clause from the main clause. In French, this punctuation is very common and makes the sentence easier to read.

If the order were reversed, the comma would often disappear:

• Ma fille préfère une tranche d’ananas quand il fait chaud.

So the comma is especially natural when the sentence begins with Quand....

Because in il fait chaud, chaud is part of a fixed weather expression.

The il here is not a real masculine subject. It is just the dummy subject used in weather expressions. So you do not change chaud to agree with a feminine noun.

You simply say:

• il fait chaud
• il fait froid
• il fait beau

No matter who is speaking or what follows, the weather expression stays the same.