Quand il fait très chaud, je préfère une tranche de pastèque à un gâteau.

In weather expressions, French often uses il fait + adjective:

• il fait chaud = it’s hot / the weather is hot
• il fait froid = it’s cold
• il fait beau = the weather is nice

So il fait très chaud is the normal way to talk about hot weather.

By contrast, c’est chaud usually means it’s hot in the sense that something is physically hot to the touch, spicy, difficult, or intense depending on context. It is not the usual basic way to describe the weather.

Fait is the il form of the verb faire in the present tense:

• faire = to do / to make
• il fait = he does / it makes

In weather expressions, French uses faire idiomatically, so you should learn il fait chaud as a set phrase rather than translating it word by word.

Yes, you could also say Lorsque il fait très chaud, though in real French that usually becomes Lorsqu’il fait très chaud because of elision.

Both quand and lorsque can mean when. The difference is mainly style:

• quand = very common, everyday
• lorsque = a bit more formal or literary

In a normal spoken sentence like this, Quand is the most natural choice.

The first part, Quand il fait très chaud, is a time clause: When it’s very hot.

French often uses a comma after this kind of introductory clause, just like English often does:

• Quand il fait très chaud, je préfère...
• When it’s very hot, I prefer...

The comma helps separate the background situation from the main statement.

The verb préférer is commonly built like this:

• préférer X à Y = to prefer X to Y

So:

• je préfère une tranche de pastèque à un gâteau
  = I prefer a slice of watermelon to a cake / to cake

This is the standard French structure. English often uses prefer X to Y too, so it matches fairly well.

A very common mistake for English speakers is trying to use something like que after préférer in this structure. For nouns, French normally uses à:

• correct: Je préfère le thé au café
• not natural here: Je préfère le thé que le café

Because à + un does not contract.

French contractions happen with:

• à + le = au
• à + les = aux

But:

• à + un = à un
• à + une = à une

So:

• à un gâteau = correct
• au gâteau would only be possible if the noun were masculine singular with le: au gâteau = to the cake

That is not what is happening in your sentence.

Both are possible, but they mean slightly different things.

• une tranche de pastèque = a slice of watermelon
• de la pastèque = some watermelon / watermelon in general

Using une tranche makes the image more specific: one slice or serving. It sounds concrete and natural in a sentence about what someone would rather eat in hot weather.

Also notice the pattern:

• une tranche de + noun
• un morceau de + noun
• un verre de + noun

So de pastèque is normal after tranche.

After expressions of quantity or unit, French usually uses de without an article:

• une tranche de pastèque
• un verre d’eau
• un kilo de pommes
• un morceau de pain

So even though pastèque by itself might often appear as la pastèque or de la pastèque, after une tranche de you just use de.

Not necessarily. In comparison structures like this, French often uses a singular noun to represent a type of food in a general way.

So à un gâteau can mean something like:

• to cake
• to a piece of cake
• to having cake

It does not always force the idea of one whole cake.

That said, this sentence is also comparing one specific serving idea to another:

• une tranche de pastèque
• un gâteau

If you wanted, you could say other versions depending on context, such as:

• je préfère la pastèque au gâteau = I prefer watermelon to cake
• je préfère une tranche de pastèque à une part de gâteau = I prefer a slice of watermelon to a slice of cake

Both can express preference.

• je préfère = I prefer
• j’aime mieux = I like better / I prefer

In this sentence, je préfère is the most direct and standard match for I prefer. It is very common in both spoken and written French.

You could also say:

• Quand il fait très chaud, j’aime mieux une tranche de pastèque qu’un gâteau.

But that changes the structure, because aimer mieux often uses que in comparisons. With préférer, the standard noun pattern is préférer X à Y.

This happens because préférer is an accent-changing verb.

Infinitive:

• préférer

Present tense:

• je préfère
• tu préfères
• il/elle préfère
• nous préférons
• vous préférez
• ils/elles préfèrent

The spelling changes to reflect pronunciation. In the forms where the following syllable is not strongly pronounced, é often becomes è.

So:

• préférer sounds roughly like pray-fay-ray
• préfère sounds roughly like pray-fehr

You do not need to master the historical reason right away; the important thing is to memorize the common present forms.

A rough pronunciation guide is:

Kahn eel fay tray shoh, zhuh pray-fehr oon trahnsh duh pahs-tek ah uh(n) gah-toh.

A few notes:

• Quand sounds like kahn
• il fait sounds like eel fay
• très chaud: the s in très is normally silent before chaud
• je often sounds like a light zhuh
• une has a rounded vowel that English does not really have
• gâteau ends with -eau, pronounced like oh

If you want to sound natural, focus especially on rhythm:

Quand il fait très chaud, je préfère une tranche de pastèque à un gâteau.

Yes. Even if the meaning has already been given, it is useful to know that French and English do not always line up word for word.

French:

• je préfère X à Y

Natural English possibilities:

• I prefer X to Y
• I’d rather have X than Y
• I like X better than Y

So this French sentence may be translated in several natural ways depending on tone, even though the grammar of the French sentence stays the same.