There is a misunderstanding here. So that "they" is translated by "ce", here are the conditions:
Presence of verb BE/ÊTRE + ARTICLE (or possessive adj.) + NOUN (with or without an adjective)
- "they are friends" => ce sont des amis
- "they are my friends" => ce sont mes amis
- "they are my great friends" => ce sont mes grands amis.
This is a far reach, but maybe will help... Think of "scribe". I think they come from the same root. B and v have extremely similar sounds in the romantic languages. And sometimes the romantic languages will add an "e" at the front of some words. Apply those two transformations, and you almost get from scribe to ecriv.