Use "c'est" before nouns with articles (un/une, le/la) or possessives (mon/ma)
The sentence in the title is the advice I got for entering "Elle est une femme qui a une forte personalité" as a translation for "She is a woman who has a strong personality". I did some research on this which I would like to share. I am putting the discussion here rather than in the sentence-specific thread because this is very fundamental grammar.
Let's see if this rule is valid, in general and in this specific case.
(1) According to the Google corpus of French literature, "Elle est une" is a rare way of starting a sentence, but not completely insignificant in comparison to "C'est une". Over the last century, "C'est une" has been about 40 times as frequent as "Elle est une". Therefore, if "Elle est une" is wrong, it would have to be a very common error.
(2) To check if it's an error, one can search for "Elle est une" in French books. And then it turns out that some of the best French authors have used the construction on occasion.
E.g. Voltaire: "Mais, dirait-on, elle est une bonne oeuvre le jour d'un saint qu'on ne fête pas ; elle est criminelle le jour d'un saint qu'on fête."
If it's good enough for Voltaire, then certainly it's good enough for me. And it's clearly not completely obsolete yet, either.
(3) I found some more detailed advice on the web that really sounds convincing: http://www.frenchtoday.com/blog/cest-versus-il-elle-est Quoting from there:
"Now, the construction “il est un…” is not wrong. But it’s now used only in formal French, so much so that it now sounds “wrong” in spoken French. And it cannot be used in all situations. In other words, it’s quite complicated, and forums go on and on about “c’est ≠ il est” because French people don’t seem to agree either :-) If you use my explanation, you won’t make mistake. It might not be the big picture, but it’s practical."
To wrap up, as a rule of thumb this rule is good. For marking translations of normal, straightforward sentences wrong if they don't follow the rule it's okay-ish. (Not because the sentences are wrong, but to encourage a more natural style of expression in French.) But when we are asked to translate an English sentence in unnatural, tortured prose to French, then the rationale for rejecting "Elle est une" simply does not apply. The French has to be unnatural and tortured in this case, and it doesn't matter that "Elle est une" contributes a little bit to that.
By the way, the background to all this is language change in French. The French verb endings keep getting more insignificant. The French personal pronouns are in the process of being fused into the verbs to form new verb prefixes that will some day replace the verb endings. And the French demonstrative pronouns are in the process of turning into new personal pronouns. But this is a very slow, gradual process. "C'est une"/"Il est une" is at the the frontier of this process.