Learn Hardcore French: Paul met la main devant sa bouche quand il est fatigué pendant la séance. - Paul puts his hand in front of his mouth when he is tired during the session.

In French, body parts are very often used with the definite article (la, le, les) instead of a possessive (sa, son, ses) when it’s obvious whose body part it is.

So:

Paul met la main devant…
literally: Paul puts the hand in front of…
but understood as: his hand.

This is parallel to common sentences like:

Il lève la main. – He raises his hand.
Je ferme les yeux. – I close my eyes.

Using sa main is not impossible, but it sounds heavier and is usually reserved for cases where there might be doubt about whose hand you mean, or where you want to emphasize that it’s his (and not someone else’s):

Il met sa main devant sa bouche, puis celle de son fils.
He puts his hand in front of his mouth, then in front of his son’s.

In your sentence, met la main is the most natural, idiomatic choice for “puts his hand”.

Why do we say devant sa bouche and not devant la bouche? Is devant la bouche also possible?

Both devant sa bouche and devant la bouche are grammatically possible.

devant sa bouche = explicitly “in front of his mouth”.
devant la bouche = usually also understood as “in front of his mouth” if the context is clear.

In practice:

Many speakers would say mettre la main devant la bouche (with la for both) as a very common pattern.
Using sa bouche makes the possessor very explicit, and you might choose it if there is any risk of confusion, or if you are contrasting different people:
- Il met la main devant sa bouche, pas devant la bouche de son voisin.
  He puts his hand in front of his own mouth, not in front of his neighbour’s.

So your sentence with sa bouche is correct and clear. A very idiomatic alternative many natives would use is:

Paul met la main devant la bouche quand il est fatigué pendant la séance.

Why is it sa bouche and not son bouche, since Paul is male?

In French, possessive adjectives (son, sa, ses) agree with the gender and number of the noun possessed, not with the person who owns it.

bouche is feminine singular → you must use sa.
So: sa bouche = his mouth or her mouth (depending on the context).

Quick reminder:

mon / ma / mes – my
ton / ta / tes – your (singular informal)
son / sa / ses – his/her/its
notre / nos – our
votre / vos – your (plural or formal)
leur / leurs – their

Examples:

Paul ferme sa bouche. – Paul closes his mouth.
Marie ferme sa bouche. – Marie closes her mouth.

Both use sa because bouche is feminine.

How is met formed from mettre, and why is there only one t?

Met is the third-person singular present tense of the verb mettre:

je mets
tu mets
il / elle / on met
nous mettons
vous mettez
ils / elles mettent

Spelling points:

The infinitive has tt: mettre.
In il met, there is only one t.
The double tt reappears in nous mettons, vous mettez, ils mettent.

Pronunciation:

met is pronounced like “mé” (same sound as mais).
The final -t is silent.

Why is the present tense used (met, est fatigué) here? Could another tense be used?

The present tense in this sentence describes a habitual action:

Paul met la main devant sa bouche quand il est fatigué pendant la séance.
= Paul does this whenever he is tired during the session.

This matches the English simple present:

Paul puts his hand in front of his mouth when he is tired during the session.

If you were talking about one specific session in the past, you would change the tense:

Hier, pendant la séance, Paul a mis la main devant sa bouche quand il était fatigué.
Yesterday, during the session, Paul put his hand in front of his mouth when he was tired.

For a future, you might say:

Demain, pendant la séance, Paul mettra la main devant sa bouche s’il est fatigué.
Tomorrow, during the session, Paul will put his hand…

Is there a difference between quand il est fatigué and lorsqu’il est fatigué? Could we say si il est fatigué?

quand il est fatigué vs lorsqu’il est fatigué

quand and lorsque are often near-synonyms meaning “when” in a temporal sense.
lorsque is a bit more formal/literary; quand is fully neutral and very common.

So you can say:

Paul met la main devant sa bouche quand il est fatigué.
Paul met la main devant sa bouche lorsqu’il est fatigué.

Both are correct; the meaning is practically the same.

si il est fatigué

Written correctly, this would be s’il est fatigué (the i of si is dropped before another i).
si means “if”, so s’il est fatigué is more conditional:
- Paul met la main devant sa bouche s’il est fatigué.
  = If he happens to be tired, he does it.

In many real situations, quand (“when(ever)”) and si (“if”) can be close in meaning, but grammatically they are different: quand marks time; si marks condition. In your original sentence, quand is the most natural choice.

What exactly does pendant la séance mean? What kind of séance is this?

Pendant la séance literally means “during the session”.

The noun séance is quite general; it just means a period of time devoted to a specific activity. Depending on context, it could be:

a class / lesson – une séance de cours, une séance de maths
a sports practice / training session – une séance d’entraînement
a therapy session – une séance de thérapie, une séance chez le psy
a film showing at the cinema – une séance de cinéma
a photo shoot – une séance photo

Without more context, you simply understand pendant la séance as during the session, leaving the exact type of session to the broader context.

Can pendant la séance be placed elsewhere in the sentence?

Yes. French word order is fairly flexible for time expressions. All of these are grammatically correct, with slightly different emphasis:

Paul met la main devant sa bouche quand il est fatigué pendant la séance.
– Neutral, the time phrase is at the end (very common pattern).
Pendant la séance, Paul met la main devant sa bouche quand il est fatigué.
– Emphasizes the time frame first: During the session, Paul…
Paul, pendant la séance, met la main devant sa bouche quand il est fatigué.
– Inserts the time info as a kind of aside; a bit more spoken or stylistic.

If you want to avoid any feeling that “quand il est fatigué pendant la séance” is too “heavy” in the middle, you can also split it slightly:

Pendant la séance, quand il est fatigué, Paul met la main devant sa bouche.

All of these mean essentially the same thing: this behavior happens during the session, when he is tired.

Why is devant used here, and not sur or en face de?

The preposition devant means “in front of” in a spatial sense:

la main devant la bouche = the hand is in front of the mouth, not necessarily touching it.

Compare with other prepositions:

sur = on, on top of, usually with contact:
- la main sur la bouche = the hand is on the mouth, covering it.
en face de = facing, opposite, often for people or objects facing each other across a space:
- Il est assis en face de Paul. – He is sitting opposite Paul.
avant is mostly “before” in a temporal or sequence sense, not used for this kind of spatial relation with body parts.

So devant sa bouche is exactly the right choice to say that the hand is placed in front of his mouth.

Is mettre la main devant la bouche an idiomatic expression? Are there other common ways to say this?

Mettre la main devant la bouche is mostly literal, but it’s also a very standard, common way to describe this gesture, especially when someone:

yawns – Il met la main devant la bouche quand il baille.
coughs – Elle met la main devant la bouche quand elle tousse.
laughs – Il rit tellement qu’il met la main devant la bouche.

There are a couple of alternative ways to express a similar idea:

Se couvrir la bouche de la main
- Paul se couvre la bouche de la main quand il tousse.
- Slightly more formal/literary; literally “covers his mouth with his hand”.
Mettre la main sur la bouche
- Emphasizes contact / covering the mouth rather than just being in front of it.
Porter la main à la bouche
- Literally “to bring the hand to the mouth”, more about the movement than the final position (often for eating, whispering, being shocked, etc.).

In your sentence, met la main devant sa bouche (or devant la bouche) is simple, clear, and idiomatic.

How would the sentence change if the person were female instead of Paul?

You mainly need to adjust:

The subject (name and pronoun).
The agreement of fatigué.

For a woman named Marie, for example:

Marie met la main devant sa bouche quand elle est fatiguée pendant la séance.

Changes:

Marie instead of Paul.
elle instead of il.
fatiguée with an -e to agree with the feminine subject elle.

Everything else (met la main, devant sa bouche, pendant la séance) stays exactly the same.