Been looking at this for a long time. Conclusion is if you want to write a sentence using a/the/my , Or making a general opinion then use c'est. Otherwise use I'll/Elle est to make a specific opinion towards something/someone if there is no a/the/my in the sentence?
This is pretty accurate but I prefer working with their French equivalents.
There are others and you can add, Monsieur, Madame and names to the list.
I am working on an lesson update which I hope will clarify the situation...
As an example you can say -
Elle est bonne nageuse
C'est une bonne nageuse
but you cannot say:
Elle est une bonne nageuse !
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