Per the explanation: "C'est instead of il/elle est
C'est is used when the noun has an article (le, la, l', un or une) or a possessive before it. Say: C'est ma femme, not: Elle est ma femme."
Okay, that great but WHEN would the noun either not have an article or a possessive in front of it?
The only time I can think of is when an adjective that comes before the noun would be in the way but surely that's not what this means??? "Elle est ma petite femme."
WHEN would the noun either not have an article or a possessive in front of it? : when you state someone's profession or with nouns used as adjectives:
il est professeur = c'est un professeur (he is a teacher)
ils sont bons amis = ce sont de bons amis (they are good friends)
elle est bonne élève = c'est une bonne élève (she is a good student)
The rule is valid with articles, possessives and demonstratives.
- she is my little wife = c'est ma petite femme (modifier: "ma")