Learn Hardcore French: Tout ce que Marie écrit sur son village parle des habitants du voisinage. - Everything Marie writes about her village is about the people in the neighborhood.

ce qui is used when the pronoun is the subject of the verb in the relative clause.
ce que is used when the pronoun is the direct object of the verb in the relative clause.

In your sentence:

Inner clause: Marie écrit tout ça sur son village.
The thing Marie writes (tout ça) is a direct object of écrit.
So you need ce que, not ce qui.

Compare:

Tout ce qui est sur la table est à toi.
“Everything that is on the table is yours.” (ce qui is subject of est)
Tout ce que tu dis m’intéresse.
“Everything that you say interests me.” (ce que is object of dis)

Why do we need ce in tout ce que? Why not just tout que Marie écrit?

You cannot say ✗ tout que Marie écrit.

Que alone must refer back to a specific noun that you’ve already named:

Le livre que Marie écrit… – “The book that Marie is writing…”

But in tout ce que, we don’t have a specific noun like le livre or la lettre; we mean “everything (that…)” in a general way. French uses ce as a neutral pronoun for “that which / what”:

ce que = “that which / what” (as object)
ce qui = “that which / what” (as subject)

So:

tout ce que Marie écrit… = “everything that Marie writes…”
Literally: “all that which Marie writes…”

Why is the verb parle in the singular and not parlent?

The grammatical subject of parle is the whole phrase:

Tout ce que Marie écrit sur son village

Even though in meaning it refers to many things Marie writes, grammatically it behaves as one singular block: “everything that Marie writes…”.

In French, tout ce que… is treated as singular, so the verb agrees in the singular:

Tout ce que tu dis me plaît.
“Everything you say pleases me.” (singular plaît)

If you wanted a clearly plural subject, you’d need a plural noun:

Toutes les choses que tu dis me plaisent.
“All the things you say please me.” (plural plaisent)

Why is it son village even though Marie is female? Shouldn’t it be sa village?

French possessive adjectives (mon, ma, mes; ton, ta, tes; son, sa, ses) agree with the gender and number of the thing owned, not with the owner.

village is a masculine singular noun.
So you must use son (masculine singular), regardless of whether Marie is female or male.

Examples:

Marie aime son village. – “Marie loves her village.”
Paul aime son village. – “Paul loves his village.”
(Same son, because village is masculine.)

Similarly:

Marie aime sa ville. (“her town”; ville is feminine)
Marie aime ses voisins. (“her neighbors”; voisins is plural)

What does sur son village mean here? Why use sur instead of de?

In this context, écrire sur means “to write about (a topic)”.

écrire sur son village = “to write about her village”

This use of sur is common when talking about the subject of a book, article, essay, etc.:

Un article sur la politique. – “An article about politics.”
Un livre sur la guerre. – “A book about the war.”

You can also find écrire (quelque chose) sur and écrire (quelque chose) au sujet de / à propos de / concernant with similar meanings. Écrire de son village is possible but sounds more like “writing from her village” in many contexts, so sur is clearer for “about (as a topic)”.

How does parle des habitants du voisinage work grammatically?

The key structure is parler de = “to talk about / to speak about”.

parler de quelqu’un / de quelque chose – “to talk about someone / something”

In your sentence:

parle de + les habitants → parle des habitants
(de + les always contracts to des)

So:

parle des habitants du voisinage = “speaks about the inhabitants of the neighborhood.”

Also note the difference:

parler à quelqu’un – to talk to someone
parler de quelqu’un – to talk about someone

Why is it des habitants du voisinage and not les habitants du voisinage?

Both are grammatically possible, but they don’t feel exactly the same.

des habitants du voisinage is indefinite:
- It suggests (some) inhabitants of the neighborhood, or the inhabitants in general as a topic, without insisting on a clearly defined group.
les habitants du voisinage is definite:
- It points more strongly to “the inhabitants of the neighborhood (all of them / that specific group)”.

In your sentence, des fits well because we’re talking about the general subject matter of her writing: her texts talk about inhabitants of the neighborhood, not necessarily every single inhabitant as a complete set.

What exactly does voisinage mean? How is it different from quartier or voisins?

Voisinage is a noun that refers to:

The area around where you live – “the neighborhood / vicinity.”
The people who live around you – “the local people / neighbors” (in a collective sense).

Comparisons:

voisinage – the neighborhood/vicinity as a general environment, or the local community.
quartier – a district / quarter / neighborhood (more concrete, often urban or administrative).
voisins – neighbors, specifically the people, as individual persons.

So les habitants du voisinage = “the inhabitants of the neighborhood / people living nearby,” focusing on people in the surrounding area.

Can a text really “speak about” something in French, like Tout ce que Marie écrit … parle des habitants…?

Yes. French, like English, often uses parler de with inanimate subjects, such as books, films, articles, etc.

Examples:

Ce livre parle de la guerre.
“This book is about the war.”
Le film parle d’une famille pauvre.
“The film is about a poor family.”
Cet article parle de pollution.
“This article is about pollution.”

Your sentence fits this pattern:

Tout ce que Marie écrit sur son village parle des habitants du voisinage.
“Everything Marie writes about her village is about / speaks of the inhabitants of the neighborhood.”

Could we move sur son village to another place in the sentence, like at the end?

You have some flexibility, but you must keep the meaning clear.

Original:

Tout ce que Marie écrit sur son village parle des habitants du voisinage.
Here sur son village clearly modifies écrit (what she writes is about her village).

If you say:

Tout ce que Marie écrit parle des habitants du voisinage sur son village.

this sounds strange or ambiguous, as if “the inhabitants of the neighborhood on her village”, which doesn’t really make sense.

Natural alternatives that keep the same meaning:

Tout ce que Marie écrit, sur son village, parle des habitants du voisinage.
(commas can show sur son village is an aside, but it still logically attaches to écrit)
Or keep the original word order, which is the clearest:
Tout ce que Marie écrit sur son village parle des habitants du voisinage.