If memory serves, I was taught the rules for comparing adjectives in fifth or sixth grade:
1. Adjectives have three degrees of comparison: positive, comparative, and superlative.
2. The comparative is formed with -er or more.
3. The superlative is formed with -est or most.
4. Short words like big and happy take -er and -est: big, bigger, biggest; happy, happier, happiest.
5. Long words, like beautiful and intelligent take more and most: beautiful, more beautiful, most beautiful; intelligent, more intelligent, most intelligent.
This simplified summary applies in most situations. Fine-tuning comes with reading experience.
Yet many speakers seem not to have learned these general rules for comparing adjectives:
He’ll go out of his way to be nice to your friends and family so he can make a good impression on them, even if it’s only because he knows that’ll make you more happy.
I am more strong than I have ever been and my clients are getting better results as well.
It did make my lawn more green. –product review
How to make your company more green
Let’s make the world more greener.
Making cars more greener
How to make your neighborhood more safe
…figuring out how to make low-income communities more safer for women.
These quotations are taken from various blogs. One could say, “Well, these aren’t professional journalists, so why be so critical?”
It seems to me that the general rules for the comparison of adjectives can be mastered by a twelve-year-old. Anyone who has completed eight years of formal education can be expected to have gotten the hang of it. But it is not only the amateur writers who get it wrong. The following is from a writer who has shared two Pulitzer prizes:
[something to do with economics] is a more strong indicator.”
Linguists might argue that dropping of the -er, -est forms is driven by the natural urge of the language towards grammatical simplification.
But “more greener” and “more safer”? I don’t think that has anything to do with evolutionary simplification of the language.