Why is it c'est pierre and not il est pierre?
Take a look at my answer to Jorge with a list of frequent errors with 'c'est/il est'.
Also, look out for a fill-in-the-blanks I worked on this very matter which might help you with this conundrum.
Carol, I note you found the answer in the discussion. Agree with you that a line noting this specific situation should be in the lesson - it is not fully covered as such presently, which explains the question appearing regularly. (I couldn’t find it covered directly in a quick search on the Lawless site either, might be there somewhere I missed).
This link may give you further understanding:
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