People often say, "that is a nice thing to do!" But when I thought this question over, this thought occurred to me: Why don't we say " That is a thing nice to do"?
My reason is as follows. We often say "boys reading under trees," not "reading boys under trees." Not that the second one is flawed or anything, but it is just the way it is. Then my question is, why don't we say "thing nice to do"?
Best Answer
Because "thing" is a (pro)noun and "nice" is an adjective that modifies the noun: Postpositive adjectives are rare in English and their use is generally formulaic; "thing" and "nice" don't meet the criteria.
As for your second paragraph: this is a different situation. "Reading" in the first sentence is a verb - the boys are doing reading; in the second sentence "reading" is an adjective - "reading" is a feature of the boys. If instead we considered it a verb the sentence (as a sentence) would be incomplete - who is reading the boys under the trees?