Both are acceptable - "simpler" is slightly preferable. One syllable: fat,fatter,fattest Two syllables: simple, simpler / more simple, simplest / most simple Three or more syllables: comfortable, more comfortable, most comfortable

Personally I would use "simple = simpler" all the time. I have never heard that explanation before though fdmaxey and looking at it now it makes perfect sense! Thank you - I may just use it with my students!