When describing people and things with être in French, you usually can't use a personal subject pronoun like elle. Instead, you must use the impersonal pronoun ce, which can also mean "this" or "that". Note that ce is invariable, so it can never be ces sont.
These pronouns aren't interchangeable. The basic rule is that you must use ce when être is followed by any determiner—for instance, an article or a possessive adjective. Note that c'est should be used for singulars and ce sont should be used for plurals.
C'est un homme. — He's a man. / This is a man. / That is a man.
Ce sont des chats. — They're cats. / These are cats. / Those are cats . C'est mon chien. — It's my dog. / This is my dog. / That's my dog.
If an adjective, adverb, or both appear after être, then use the personal pronoun.
Elle est belle. — She is beautiful. (Or "It is beautiful.")
Il est très fort. — He is very strong. (Or "It is very strong.")
Ils sont calmes. — They are calm.
As you know, nouns generally need determiners, but one important exception is that professions, nationalities, and religions can act as adjectives after être. This is optional; you can also choose to treat them as nouns.
He is a doctor. — Il est médecin. / C'est un médecin.
However, c'est should be used when using an adjective to make a general comment about (but not describe) a thing or situation. In this case, use the masculine singular form of the adjective.
C'est normal ? — Is this normal?
Non, c'est étrange. — No, this is strange.
OLR, here is the link Ripcurlgirl got that information from: https://www.duolingo.com/skill/fr/Gallicism%3Ail-elle-OR-c-ce/tips-and-notes
Good to know, thanks.
But Will posted the wrong link - it is:
His link still returns to the home page.
I think that is because you are on different versions of the tree. The old tree has the skill "Être / Avoir" whereas the new has the skill "Gallicism". There is still an ongoing A/B test where some users have the old tree, and some have the new, so whether you can access a certain skill URL most likely depends on whether you have it in your current tree version.
Does anyone know what the rule is on pronouncing the final e or es of a word? Is there a slight euh sound at the end of both?