Can we say "John is the stiffest of Rachel and Mark.", OR "John is more stiffer than Rachel and Mark."?
Which one is better?
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Sign up to join this communityBoth are wrong. A person can only be 'the [adjective]est' of a group which includes them, so if Rachel and Mark are John's brother and sister you could say 'John is the stiffest one of the family'. Otherwise it has to be 'John is stiffer than Rachel and/or Mark'.
When we compare more than two things we must use the superlative degree form. In your case, it is 'the stiffest'. The other /comparative degree form 'stiffer' / is grammatically wrong. The double comparison /'more stiffer'/ is impossible.
more stiffer
is not accepted form, refrain from using it.