Learn Hardcore French: Paul garde un petit morceau de roche dans sa poche comme souvenir. - Paul keeps a small piece of rock in his pocket as a souvenir.

Garder usually means to keep, to hold onto, sometimes to look after or to guard.
In this sentence, Paul garde ... means Paul keeps ... (as in, he doesn’t throw it away; he keeps it with him).

Conjugation of garder in the present tense:

je garde
tu gardes
il / elle / on garde
nous gardons
vous gardez
ils / elles gardent

So garde matches Paul (3rd person singular).

Why is it un petit morceau de roche and not une petite morceau de roche?

Because morceau is masculine, and adjectives and articles must agree in gender and number with the noun they modify.

morceau = a piece / a bit / a chunk (masculine singular)
Therefore:
- article: un (masculine singular)
- adjective: petit (masculine singular form)

If it were a feminine noun, you’d see une petite ..., for example:

une petite pierre (a small stone) – pierre is feminine.

Here, roche is feminine, but it’s not the main noun of the phrase; morceau is. The structure is:

un petit morceau (main noun + its article + its adjective)
de roche (complement: a piece of rock)

What is the role of de in morceau de roche? Why not de la roche or en roche?

Here de introduces what the piece is made of or taken from: a piece of rock.

un morceau de roche = a piece of rock (a chunk of that material, in general)
This is similar to un verre de jus (a glass of juice), un morceau de pain (a piece of bread).

Differences:

un morceau de roche: a piece of rock (general material, not specific rock).
un morceau de la roche: a piece of the rock (a specific rock that has already been identified in context).
en roche would usually mean made of rock in an adjectival sense (e.g. un mur en roche = a wall made of rock), but with morceau, French normally uses de, not en.

So de here is like of in English, linking a piece/quantity to the material.

Why does petit come before morceau when I learned that French adjectives normally come after the noun?

Many French adjectives do come after the noun, but there is a common group that usually comes before. A classic memory trick is BAGS (Beauty, Age, Goodness, Size):

Beauty: joli, beau
Age: jeune, vieux
Goodness: bon, mauvais
Size: grand, petit, gros, etc.

Petit is a size adjective, so it usually comes before the noun:

un petit morceau
une petite maison
un grand arbre

Most other adjectives (colors, shapes, nationalities, many descriptive adjectives) come after:

une roche noire
un morceau irrégulier

Why is it dans sa poche and not dans son poche or dans la poche?

Poche is feminine singular:
- la poche = the pocket (feminine)
- So the feminine singular possessive adjective is sa, not son:
  - sa poche = his pocket / her pocket
In French, the possessive (son, sa, ses) agrees with the thing possessed, not the person who owns it:
- sa poche = the owner could be Paul (male) or Marie (female); we only know the pocket is feminine.
Why not la poche?
- dans sa poche = in his pocket (specifically Paul’s pocket).
- dans la poche would mean in the pocket (more generic or context-dependent, not clearly Paul’s own pocket).

So dans sa poche is the natural way to say in his pocket here.

Could I say dans son poche if it’s a man’s pocket? Doesn’t son mean “his”?

No, not here. Again, son / sa / ses agree with the noun that follows, not with the gender of the owner.

poche is feminine → must use sa poche, regardless of whether the owner is male or female.
son is used:
- before a masculine singular noun: son sac (his/her bag)
- or before a feminine noun starting with a vowel sound to avoid awkward pronunciation: son amie (his/her female friend).

So:

Paul garde un billet dans sa poche. (poche = feminine → sa)
Paul garde un billet dans son sac. (sac = masculine → son)

What is the difference between dans sa poche and other prepositions like à sa poche or en sa poche?

For physical location in his pocket, the normal, correct choice is dans sa poche.

dans sa poche = in his pocket, literally inside his pocket.
en sa poche is old-fashioned or literary and basically not used in modern spoken French.
à sa poche does not work for physical location; it might appear in some expressions (e.g., faire mal à la poche = be expensive) but not to mean in his pocket.

So in everyday modern French:

Use dans for being physically in / inside a container:
- dans la poche, dans le sac, dans la boîte.

Why is it comme souvenir and not pour souvenir, en souvenir, or comme un souvenir?

Comme souvenir here means as a souvenir or as a keepsake.

comme = as, in the role of
- Il garde ça comme souvenir. = He keeps that as a souvenir.

About the article:

No article (comme souvenir) is very common in French when you describe the function or role of something:
- Travailler comme professeur = to work as a teacher.
- Le garder comme souvenir = keep it as a souvenir.

Alternatives:

comme un souvenir:
- Also possible, but it slightly stresses one souvenir among others or gives it a bit more individuality.
pour souvenir:
- This sounds wrong in modern French; you might see pour souvenir de in some fixed or older expressions, but it’s not standard here.
en souvenir usually appears as en souvenir de = in memory of:
- Il garde ça en souvenir de son voyage. = He keeps that in memory of his trip.

So in your sentence, comme souvenir is the most natural and neutral way to say as a souvenir.

If Paul were a woman, would sa poche change? For example, with Pauline?

No, it would stay the same, because the possessive agrees with poche, not with the person.

Paul tient un livre dans sa main.
Pauline tient un livre dans sa main.

In both, main is feminine, so sa main is used for both Paul and Pauline.

Similarly:

Paul garde un petit morceau de roche dans sa poche comme souvenir.
Pauline garde un petit morceau de roche dans sa poche comme souvenir.

The sentence is identical; context or prior mention tells you whether sa means his or her in English.

Could I use other words than roche here, like rocher, pierre, or caillou? Are they the same?

They are related but not identical; each has its nuance.

roche (feminine):
- General term for rock as a material or mass (more geological/scientific or generic).
- Fits well with morceau de roche = a piece of rock (material).
rocher (masculine):
- Usually a large visible rock, a boulder, a rocky formation.
- un rocher is something you might see sticking out of the sea or on a mountain.
- un morceau de rocher would sound like a chunk broken off a big boulder.
pierre (feminine):
- A stone (often smaller than a rocher), or a stone used in construction or as a gemstone.
- une petite pierre = a small stone / pebble.
- un morceau de pierre is also possible.
caillou (masculine):
- A small stone, a pebble (informal, everyday word).
- un petit caillou = a little stone/pebble (very natural in everyday French).

So your sentence could be adapted, for example:

Paul garde un petit caillou dans sa poche comme souvenir. (a little pebble)
Paul garde une petite pierre dans sa poche comme souvenir. (a small stone)

The original morceau de roche sounds a bit more like a piece of rock broken off something larger, slightly more neutral or descriptive.

How do you pronounce the whole sentence, including any liaisons?

One standard French pronunciation (IPA) is:

[pol gaʁd œ̃ pəti mɔʁso də ʁɔʃ dɑ̃ sa pɔʃ kɔm suvəniʁ]

Breakdown with notes:

Paul → [pol]
- Like pol in Poland (without the final “d”).
garde → [gaʁd]
- g as in go.
- French r in the throat.
- Final -e is silent, but -d is pronounced: gard.
un → [œ̃]
- Nasal vowel; somewhat like uh in burn, but nasalized, no n sound at the end.
petit → [pəti]
- Final t is silent.
- Stress is light and mostly toward the last syllable: pe-TI.
morceau → [mɔʁso]
- Final -eau = [o] sound.
de → [də]
- Very light de, almost like duh but shorter.
roche → [ʁɔʃ]
- Final -e mostly silent; ends in the -sh sound.
dans → [dɑ̃]
- Nasal an sound; no clear n at the end.
sa → [sa]
- Like sa in salsa.
poche → [pɔʃ]
- Same ending sound as roche.
comme → [kɔm]
- Like kom.
souvenir → [suvəniʁ]
- Stress toward the end: sou-ve-NIR.
- Final r is pronounced (French r).

There are no required liaisons between these particular words in normal speech; you would usually pronounce them separately as written above.