Learn Hardcore French: J’utilise le micro-ondes pour chauffer la soupe, mais Marie allume le four pour cuire le pain. - I use the microwave to heat the soup, but Marie turns on the oven to bake the bread.

French elides the vowel of certain short words before a following vowel (or mute h). So Je becomes J’ before utilise. The same happens in J’aime, J’allume, J’habite. No elision occurs before an aspirated h (e.g., je harcèle).

Why does micro-ondes end with an s if it’s singular? Is the hyphen/spelling right?

The appliance name is a shortening of four à micro-ondes. The word ondes (“waves”) stays plural inside the compound, even when the whole noun is singular: un/le micro-ondes.
Plural looks the same: des micro-ondes.
The hyphen is standard: micro-ondes. You’ll hear un micro-onde (without s) in speech, but dictionaries and style guides recommend the s.

Is “J’utilise le micro-ondes pour chauffer la soupe” idiomatic, or is there a more natural way to say it?

It’s correct, but common, snappier options are:

Je chauffe la soupe au micro-ondes.
If it’s already cooked: Je réchauffe la soupe au micro-ondes.
With “put”: Je mets la soupe au micro-ondes pour la chauffer.

What’s the difference between chauffer and réchauffer?

chauffer = to heat (possibly from cold): chauffer de l’eau.
réchauffer = to heat again (something already cooked/heated): réchauffer la soupe / des restes. For soup that was made earlier and is now cold, réchauffer is the usual choice.

“Cuire,” “faire cuire,” or “cuisiner”: which one should I use with bread?

To “bake bread”: cuire du pain or, more commonly in everyday speech, faire cuire du pain.
When the food is the subject doing the cooking: Le pain cuit au four.
Cuisiner means “to cook/prepare dishes” in general; you don’t normally say cuisiner du pain. Note: Many speakers prefer faire cuire when a person is the subject (to avoid clunky forms like “le cuire”).

Does allumer le four mean “turn on” or “light”? What about préchauffer?

allumer le four = turn the oven on (gas or electric) / light it (for gas).
préchauffer le four (à 180 °C) = preheat to a given temperature.
You may also hear mettre le four à 180 °C (set the temperature). Cooks typically say: Préchauffe le four à 180 °C.

Why is it pour + infinitive? Could I use afin de or à?

pour + infinitif expresses purpose: pour chauffer… / pour cuire… It’s the default.
afin de + infinitif also expresses purpose but is a bit more formal: afin de chauffer…
à + infinitif is not used to express purpose here (don’t say à chauffer). You will see it in set phrases like prêt à cuire (“ready to cook”).

Why is it la soupe (definite article)? When would I use de la soupe?

la soupe = “the soup” (a specific soup in context) or soup as a category: La soupe est chaude.
de la soupe = “some soup” (an unspecified quantity): Je veux de la soupe. In your sentence, la soupe suggests a particular soup they have.

Why le pain rather than du pain?

le pain can mean a specific loaf already known in context, or bread as a category (“bread in general”).
If Marie is baking “some bread” (an unspecified amount), everyday French would more likely say cuire du pain / faire cuire du pain. So both are possible; it depends on whether you mean a known loaf, bread in general, or some bread.

If I replace “la soupe” with a pronoun, where does it go?

With an infinitive of purpose, object pronouns go before the infinitive:

J’utilise le micro-ondes pour la chauffer. (“la” = la soupe) With negation:
Je n’utilise pas le micro-ondes pour la chauffer. For bread you can say:
Marie allume le four pour le cuire, though many prefer the smoother … pour faire cuire le pain.

Is the comma before mais correct in French?

Yes. French often places a comma before coordinating conjunctions like mais (“but”) to mark the pause: …, mais … This is more common in French than in English.

Any tricky pronunciation points here?

J’utilise: final -se is pronounced
micro-ondes: nasal vowel in -ondes; the final s is silent.
chauffer: ch = [ʃ] (sh sound).
cuire: starts with [kɥ] (like “k” plus a tight “w”).
four and pour: [fuʁ], [puʁ] with the French r.
No typical liaison after mais here; read it smoothly as two chunks.

What are the genders of the nouns?

le micro-ondes: masculine
la soupe: feminine
le four: masculine
le pain: masculine

Could I say Je me sers du micro-ondes instead of J’utilise le micro-ondes?

Yes. se servir de (“to make use of”) is very common and slightly more colloquial:

Je me sers du micro-ondes pour réchauffer la soupe. Meaning is the same as J’utilise le micro-ondes…

Should it be au four or dans le four?

Method/instrument: cuire au four (“to bake,” literally “cook by oven”).
Movement: mettre au four (“put in the oven”).
dans le four is literal “inside the oven” and is fine with movement too (mettre dans le four), but for cooking method French strongly prefers au four.