Learn Hardcore French: La raison pour laquelle elle écrit à la mairie est simple. - The reason for which she is writing to the town hall is simple.

So la raison pour laquelle elle écrit à la mairie = the reason for which she writes to the town hall (= the reason why she writes to the town hall).

Why is it laquelle and not lequel, qui, or que?

Because:

Agreement in gender and number
- raison is feminine singular (la raison),
- so we must use the feminine singular form laquelle (not lequel, which is masculine).
The preposition “pour”
In French, when a relative pronoun follows a preposition (pour, avec, à, de, etc.), we normally use lequel / laquelle / lesquels / lesquelles, not qui or que.
So:
- ✅ la raison pour laquelle…
- ❌ la raison pour qui… (sounds wrong here)
- ❌ la raison pour que… (different structure; mostly used with verbs of will/necessity, not “reason”)
- ❌ la raison que elle écrit… (wrong: que can’t follow pour like this)

So the correct relative construction with pour + a feminine singular noun is pour laquelle.

Why can’t I just say la raison qu’elle écrit à la mairie?

Because in that structure, que would have to be the direct object of the verb écrire, and that makes no sense here.

écrire works like this:
- écrire quelque chose = to write something (direct object)
- écrire à quelqu’un = to write to someone (indirect object with à)
- écrire quelque chose à quelqu’un = to write something to someone

In elle écrit à la mairie, the thing being written (the letter, email, etc.) is understood but not expressed, and à la mairie is the indirect object (who she writes to).

The word raison is not the object of écrire at all; it is linked by the idea “the reason for which she writes”. So we need:

pour laquelle (for which [reason])

and not que, which would make raison look like a direct object of écrire.

Could I say la raison pourquoi elle écrit à la mairie instead?

You will hear and read la raison pourquoi in everyday French, but:

many grammar books and teachers recommend la raison pour laquelle as the standard / more correct form;
la raison pourquoi is often considered less formal or less elegant, and some people label it incorrect (even though it’s common in practice).

So:

In careful / written French: prefer la raison pour laquelle.
In casual spoken French: you may hear la raison pourquoi, but it’s safer as a learner to stick with pour laquelle.

What is the difference between la raison pour laquelle… and pourquoi?

They both relate to why, but they’re used differently:

pourquoi is used mostly in questions:
- Pourquoi écrit-elle à la mairie ? = Why is she writing to the town hall?
la raison pour laquelle… is used to introduce an explanation inside a larger sentence:
- La raison pour laquelle elle écrit à la mairie est simple.
  The reason why she is writing to the town hall is simple.

You can think of it like this:

pourquoi = why?
la raison pour laquelle = the reason why… (as a noun phrase).

Why is it à la mairie and not au mairie?

Because mairie is feminine:

la mairie = the town hall / city hall
à + la mairie = à la mairie (no contraction)

au is a contraction of à + le, which is only used with masculine nouns:

à + le cinéma → au cinéma
à + le parc → au parc

Since mairie is feminine, we must say à la mairie, never au mairie.

What exactly does mairie mean? Is it the same as “town hall”?

La mairie usually refers to:

The building where the local council / mayor works
→ town hall / city hall.
The municipal administration itself as an institution
→ the town/city council, the local authorities.

So elle écrit à la mairie can mean:

she is writing to the town hall (as a place),
or more naturally in English: she is writing to the local authorities / town council.

In larger cities you might also see l’hôtel de ville, which is another term for the main town hall building.

Why is it elle écrit à la mairie and not elle écrit la mairie?

Because in French:

écrire quelqu’un is not used to mean “write to someone”.
The normal pattern is:
- écrire à quelqu’un = to write to someone
  - écrire à la mairie
  - écrire à son ami
  - écrire à ses parents

When you add the thing being written:

écrire quelque chose à quelqu’un
- Elle écrit une lettre à la mairie. = She is writing a letter to the town hall.

So:

✅ elle écrit à la mairie
❌ elle écrit la mairie (wrong structure)

Why is it est simple and not c’est simple?

Because the grammatical subject of the sentence is clearly la raison pour laquelle elle écrit à la mairie.

Subject: La raison pour laquelle elle écrit à la mairie
Verb: est
Complement: simple

So we say:

La raison pour laquelle elle écrit à la mairie est simple.

We would use c’est simple in a different structure, for example:

Pourquoi écrit-elle à la mairie ?
C’est simple : elle veut se plaindre.
(Why is she writing to the town hall? It’s simple: she wants to complain.)

Here, because we keep the full noun phrase as subject, we stick with est simple.

Is there anything special about the word order pour laquelle elle écrit à la mairie compared with English?

Yes. The main differences:

Preposition position
In English, you can say:
- the reason (which) she writes to the town hall for
  (preposition for at the end)
In French, the preposition must stay before the relative pronoun:
- la raison pour laquelle elle écrit à la mairie
  You cannot move pour to the end as you do in English.
No relative pronoun dropping
In English you can omit which:
- the reason she writes to the town hall
In French you must keep a relative pronoun:
- la raison pour laquelle elle écrit à la mairie (you cannot just drop laquelle)

So French is stricter: preposition + relative pronoun must stay together, and the pronoun cannot be omitted.