When does the past participle change spelling?

Question

Why has 'Mort' as the past participle of Mourir changed spelling to 'mour' ? I understand adding the 's' to make it match.

Even in the 'learn & discuss; section, it show the past participle as 'mort'. is this an error ?

Accepted Answer

Hi Phil,
"ATTENTION:Because they're using être as auxiliary verb, their past participles have to agree with the subject of the verb."
Above copied from the lesson.
This verb takes être only as auxiliary so the past participle has to match the subject.
So if the subject is male then the past participle will be "mort" and if the subject is female then the past participle is "morte"
Hope this helps to explain for you.
Bonne continuation.
Jim