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

Pour laquelle literally means for which (one).

• la raison = the reason (feminine)
• pour = for
• laquelle = which one (feminine singular form of lequel)

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).

Because:

1. 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).

2. 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.

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.

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.

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).

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.

La mairie usually refers to:

1. The building where the local council / mayor works
   → town hall / city hall.
2. 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.

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)

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.

Yes. The main differences:

1. 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.

2. 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.