I've found a cited example of incorrect usage in Bill Bryson's "Troublesome Words" which I don't understand because he doesn't explain it, and even going back to the source (Ernest Gowers' "Plain Words") doesn't help.
Apparently, the sentence "nearly twice as many people die under 20 in France THAN in Great Britain" is wrong because it should read "AS in Great Britain".
Could someone please tell me why this is wrong and, if so, explain how you know when to use "than" and when to use "as"? I'm driving myself mad second-guessing myself every time I use "than" in case I should be saying "as"! Thank you.