If I say 'more simple', is it incorrect?
Good afternoon, Torpes.
(1) You have asked a great question.
(2) Sometimes you have a choice with two-syllable adjectives.
(a) You may use "-er" or "more."
(b) Sometimes speakers or writers choose the one that they think
sounds "better" in a particular sentence.
(3) Mr. L. G. Alexander in his reliable Longman English Grammar says
both "simpler" and "more simple" are correct.
(a) Since you are a learner, it might be a good idea to use the -er form
until you are more acquainted with English.
By the way, many books tell us that sometimes you HAVE TO use "more."
For example: He is more lazy than stupid.
One may NOT say: He is lazier than stupid.
Have a nice day!