Learn Hardcore French: Le brouillard disparaît quand le soleil brille.

Unfortunately, you usually can’t reliably guess the gender from the meaning. You just have to learn brouillard (m.) as vocabulary, ideally together with its article: le brouillard.

A few helpful points:

Le brouillard = “fog” in general (mass noun, generic).
If you want to say “some fog / foggy conditions” you might see du brouillard (partitive article, still masculine).

But the reason it’s le and not la is simply that brouillard is a masculine noun in French.

Why is it le brouillard and not just brouillard without an article, like English “Fog disappears…”?

French almost always needs an article in front of a noun, even when English doesn’t use one.

Here, le brouillard is used in a general/generic sense (“fog in general”), so French uses the definite article:

Le brouillard disparaît = Fog (in general) disappears…

Compare:

Le brouillard est dangereux. = Fog is dangerous. (general statement)
Il y a du brouillard ce matin. = There is (some) fog this morning. (specific amount/occurrence)

In English, zero article (“Fog disappears…”) is fine, but in French you typically choose le, la, les, du, de la, etc., depending on the meaning. Here the generic rule uses le.

What is the infinitive of disparaît, and how is it conjugated?

The infinitive is disparaître (“to disappear”).

Disparaît is:

3rd person singular
present tense
indicative mood

Conjugation (present indicative):

je disparais
tu disparais
il / elle / on disparaît
nous disparaissons
vous disparaissez
ils / elles disparaissent

Notice:

The stem changes spelling (dispar- → disparaiss-) in the plural forms.
The circumflex in disparaît marks the standard spelling (though disparait without the circumflex is also accepted in modern spelling).

Why is it the present tense (disparaît, brille) even though this sounds like a general rule?

In French, the present indicative is used not only for actions happening right now, but also for:

General truths / scientific facts
Habits, regular events, natural laws

So:

Le brouillard disparaît quand le soleil brille.
= Whenever this situation occurs in general, this is what happens.

This is parallel to English, which also uses the present simple for general truths:

“Fog disappears when the sun shines.”

No special tense is needed; the normal present tense does the job for general statements in French.

Could I use the future tense instead? For example: Le brouillard disparaîtra quand le soleil brillera?

Yes, but the meaning changes slightly.

Le brouillard disparaît quand le soleil brille.
General rule / usual phenomenon.
Le brouillard disparaîtra quand le soleil brillera.
You’re talking about a specific future situation:
“The fog will disappear when the sun shines (later).”

Important note:
For future events, French normally uses future + future after quand:

Quand il arrivera, je partirai. = When he arrives, I will leave.

English would say “When he arrives, I will leave” (present + future), but French prefers to match tenses: future in both clauses for a future context.

What’s the difference between quand and lorsque? Could I say lorsque here?

Both quand and lorsque can mean “when” in a temporal sense, and you can say:

Le brouillard disparaît lorsque le soleil brille.

Differences:

Quand is more frequent and neutral in everyday speech.
Lorsque is slightly more formal or literary.
In many contexts they’re interchangeable; in this sentence, both are correct.

Neither quand nor lorsque automatically requires the subjunctive here, because we’re expressing a real, factual condition, not something hypothetical or emotional.

Why is the clause order Le brouillard disparaît quand le soleil brille and not the other way around? Can I put quand at the beginning?

You can absolutely put the quand-clause at the beginning:

Quand le soleil brille, le brouillard disparaît.

Both orders are correct:

Main clause + quand-clause (no comma):
Le brouillard disparaît quand le soleil brille.
Quand-clause + main clause (comma after the first clause):
Quand le soleil brille, le brouillard disparaît.

So the word order is flexible here; just remember the comma when the quand clause comes first.

Why is it le soleil brille and not something like il fait du soleil?

Both are possible, but they have slightly different uses and nuances:

Le soleil brille.
Literally: “The sun is shining.”
The subject is le soleil, and the verb is briller (“to shine”). This describes what the sun itself is doing.
Il fait du soleil.
Impersonal weather expression, like “It’s sunny.”
Here il is an impersonal subject (doesn’t refer to a person or thing), faire is used in a “weather” sense, and du soleil means “some sun / sunny conditions.”

In this sentence, we’re talking about a physical phenomenon (fog vanishing when the sun shines), so le soleil brille is very natural and direct.

Can weather be expressed with a noun as the subject like le brouillard and le soleil, or must I always use il as in il fait froid, il pleut, etc.?

You can do both, depending on the structure:

Impersonal “il” + verb (very common for weather):
- Il pleut. = It’s raining.
- Il neige. = It’s snowing.
- Il fait froid. = It’s cold.
- Il y a du brouillard. = There is fog / It’s foggy.
Weather noun as grammatical subject:
- Le brouillard disparaît. = The fog disappears.
- La pluie tombe. = The rain is falling.
- Le vent souffle. = The wind is blowing.
- Le soleil brille. = The sun is shining.

In your sentence, le brouillard and le soleil are just normal subjects of the verbs disparaître and briller. That’s perfectly correct, especially when describing what happens to these elements (fog, sun, wind, etc.).

How do I pronounce brouillard and disparaît? Are any final letters silent?

Yes, several letters are silent.

brouillard:

IPA (approx.): /bʁu.jaʁ/
brou- sounds like “broo”.
-ill- before a is pronounced like y in “yard”: brou-ya-.
Final -rd: the r is pronounced in French, but the d is silent.
So you hear something like: broo-yar.

disparaît:

IPA (approx.): /dis.pa.ʁɛ/
dis- as in “deece”.
-pa- like “pa” in “pasta”.
-raît: the t is silent; sounds like rè (like “reh”).
Overall: dees-pa-reh (with a French r).

Silent letters at the end (like t in disparaît, d in brouillard) are very common in French.

Why is there a circumflex accent in disparaît? Does it change the pronunciation?

The circumflex â, ê, î, ô, û often has historical reasons. In many words, it marks where an old s used to be (e.g., forêt ← forest).

In disparaître, historically there was a different spelling, but today:

Disparaître / disparaît is the traditional spelling.
Modern orthography reforms also allow disparaitre / disparait (without the circumflex) in some cases.

Pronunciation:

With or without the circumflex, disparaît is pronounced the same in everyday speech: /dis.pa.ʁɛ/.
So here, the circumflex is mostly an orthographic/conventional mark, not a pronunciation change, at least in modern standard French.

Could this sentence use the subjunctive after quand?

No, not in this context.

Quand here introduces a real-time / habitual condition (whenever the sun shines, this actually happens).
For real, factual, or habitual time clauses, French uses the indicative, not the subjunctive.

So:

Le brouillard disparaît quand le soleil brille. ✅ (indicative)
A subjunctive like “… quand le soleil brille” → brille is indicative, and it’s correct; a subjunctive brille would not be triggered by quand in this meaning.

The subjunctive is usually used after certain conjunctions that express doubt, necessity, intention, fear, etc. (bien que, pour que, afin que, avant que, quoique…), which is not the case here.