More questions about de vs de la

Question

I am still trying to figure out when to use "de" vs du or de la. In the sentence "qui a servi de résidence papal....." Why is it not "de la" as this seems to be a very specific thing and not a general category.

Thanks in advance

Céline Pickard · Accepted Answer

Bonjour Jill,
That's an excellent question!
In the sentence "qui a servi de résidence papale...", "de" is used because it's part of a fixed expression: 
servir de [quelque chose] = to serve as [something]
So, "de" in this context doesn't act as a partitive article (du/de la), but as part of the verbal expression.
Take a look at the following examples:
Cela peut servir d'exemple = This can serve as an example
Le hangar sert de garage = The shed serves as a garage
Cette triste aventure lui a servi de leçon = This sad adventure taught him a lesson 
I hope this is helpful.
Bonne journée !