"She is a girl and she is calm."
Translation:C'est une fille et elle est calme.
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".
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 (as it is in this sentence → "She is a girl") or a possessive adjective. Hence, C'est une fille.
If an adjective, adverb, or both appear after être, as is the case with "she is calm", then use the personal pronoun → elle est calme.
Yes. The rule is that "il/elle est" and "ils/elles sont" change to "c'est" and "ce sont" before a modified noun, that is, a noun preceded by a modifier. A modifier can be:
- an article: un, une, des, le, la, l', les
- a number: un, deux...
- a possessive adjective: mon, ton, son, ma, ta, sa, notre, votre, leur, mes, tes, ses, nos, vos, leurs
- a demonstrative adjective: ce, cet, cette, ces
These articles go into more detail and are worth a read.
Also, the Tips and Notes for the Gallicism skill (click the lightbulb icon when you open the skill) give more information.