Learn Hardcore French: Au matin, sa fièvre baisse un peu, mais la fatigue reste forte. - In the morning, her fever goes down a little, but the tiredness remains strong.

au matin = in the morning / by morning, often a bit more literary, and often about a specific morning following a previous event in a story:
- Au matin, sa fièvre baisse… → By (the next) morning, his/her fever goes down…
le matin = in the morning(s), more neutral and can be:
- habitual: Le matin, je bois du café. (In the mornings, I drink coffee.)
- or one specific morning, depending on context.
dans la matinée = during the morning / sometime in the morning, stressing the period, not the simple point in time:
- Dans la matinée, sa fièvre baisse. → During the morning, his/her fever goes down.

So au matin here fits a narrative style: something happened (fever was high), and by the morning it has gone down a bit.

What exactly is au here? Is it a preposition plus an article?

Yes. au is the contraction of:

à (preposition: at / to / in)
- le (definite article: the, masculine singular)

So:

à + le matin → au matin

You get the same pattern with many masculine nouns:

au cinéma = à + le cinéma
au travail = à + le travail

Why do we say sa fièvre and not son fièvre if we are talking about a man?

In French, possessive determiners (son, sa, ses) agree with the gender and number of the noun, not the gender of the owner.

fièvre is feminine singular → you must use sa:
- sa fièvre = his fever or her fever, depending on context.

Compare:

son livre (book = masculine) → his book or her book
sa voiture (car = feminine) → his car or her car

So sa fièvre can describe a man or a woman; French does not mark the owner’s gender here.

Is baisse here a noun or a verb, and how is baisser used with things like fevers?

In this sentence, baisse is a verb:

It’s the 3rd person singular present of baisser:
- (sa fièvre) baisse → (his/her fever) goes down / drops / decreases

Common patterns with baisser and things like fevers:

La fièvre baisse. → The fever is going down.
Le prix du pétrole baisse. → The price of oil is going down.

You can also make it transitive when something causes the reduction:

Ce médicament fait baisser la fièvre. → This medicine lowers the fever.

Here it’s intransitive: the fever itself is the subject that goes down.

Where can I put un peu in this sentence? Could I say Au matin, un peu sa fièvre baisse?

No. Au matin, un peu sa fièvre baisse is incorrect.

In neutral French, un peu goes after the conjugated verb:

Au matin, sa fièvre baisse un peu… ✅

Other possibilities:

Sa fièvre baisse un peu au matin. ✅ (time expression at the end)
Au matin, sa fièvre baisse légèrement. ✅ (using another adverb)

You normally do not put un peu in front of the subject like in English “a little, his fever goes down”. In French, that sounds unnatural unless you are doing very marked, poetic word order.

The sentence starts with Au matin and then sa fièvre baisse. Is sa here the subject pronoun like il or elle?

No. sa is not a subject pronoun. It is a possessive determiner (like his / her before a noun).

Subject of the verb baisse: sa fièvre
- fièvre = the noun
- sa = possessive determiner that modifies fièvre

French subject pronouns would be il, elle, on, etc., but here the subject is the noun phrase sa fièvre.

Could we also put the time expression at the end and say Sa fièvre baisse un peu au matin? Does the meaning change?

Yes, you can say:

Sa fièvre baisse un peu au matin. ✅

The basic meaning is the same. The difference is in emphasis and style:

Au matin, sa fièvre baisse un peu…
- Slight emphasis on the time; common in narrative writing.
Sa fièvre baisse un peu au matin.
- Slightly more neutral or conversational; the key information seems to be the fever, and the time is added as extra detail.

Both are correct.

In English we often don’t put a comma before but. Is the comma before mais obligatory in French?

In standard written French, you normally do put a comma before mais when it links two clauses:

…, mais la fatigue reste forte.

General rule: with the coordinating conjunctions mais, ou, et, donc, or, ni, car, a comma is usually put before them when they connect two full clauses. With mais, the comma is especially systematic.

Why do we use reste in la fatigue reste forte instead of just la fatigue est forte?

rester means to remain / to stay. It expresses continuation, not just a state.

la fatigue est forte → the fatigue is strong (no idea if it just became strong or has been strong)
la fatigue reste forte → the fatigue remains strong (it was already strong and continues to be)

In this context, it contrasts nicely with sa fièvre baisse:

Fever changes (goes down).
Fatigue does not change; it stays strong.

Why is the adjective forte used with fatigue? Could we also say la fatigue reste grande or importante?

With fatigue, the natural, common collocations are:

une forte fatigue → strong / intense fatigue
une grande fatigue → also used; great / deep fatigue
une fatigue importante → more formal/medical; significant fatigue
une fatigue intense → intense fatigue

In everyday French, fort(e) is very common to mean strong/intense when talking about sensations or states:

une douleur forte (strong pain)
un vent fort (strong wind)

So la fatigue reste forte sounds idiomatic and vivid: the tiredness is still very intense.

Why is it forte with an -e at the end?

Adjectives in French agree with the gender and number of the noun.

fatigue is feminine singular → adjective must be féminine singulier too:
- fort (masculine singular)
- forte (feminine singular)

So:

un vent fort (vent = masculine)
une fatigue forte (fatigue = feminine)

Why is it la fatigue and not sa fatigue, like sa fièvre earlier in the sentence?

Both la fatigue and sa fatigue are possible, but there is a nuance:

sa fatigue would insist a bit more on that person’s own particular fatigue.
la fatigue can sound a little more generic or objective, like the fatigue (that he/she feels) as a state, not so much as a possession.

French often uses the definite article with abstract or general states:

la peur, la faim, la joie, la fatigue

Here, context usually makes it clear whose fatigue it is (the same person with the fever), so la fatigue is enough and sounds natural.

Both verbs are in the present tense. Is this describing something that happens habitually every morning, or just once?

The présent in French can express:

A general/habitual fact:
- In the mornings, his fever goes down a bit, but the fatigue stays strong (every day).
A single event in a narrative (historical or “vivid” present):
- By morning, his fever goes down a bit, but the fatigue remains strong (that particular morning in the story).

Without wider context, both readings are possible. The use of au matin often feels slightly narrative, so it may lean toward “that” morning in a story, but only the broader text decides.