"Those who act unhappily all the time"

Prager, are you joking?

Okay, you're probably not. "Unhappily" is an adverb and describes how they're acting, right?

Wrong.

"To act" in this case is a copulative verb. Or, since the word "copulate" is a naughty one, you may have learned the term as "linking verb", which means that the subject and its predicate nominative are connected in a way that links them, so that you have a noun or pronoun before the verb and an adjective or adjectival noun after the copulative verb.

Thus, John acts smart. John is modified by the word smart, and we don't use an adverb "smartly" to modify the verb "acts". "John acts smartly" is just evil.

Don't get it yet? Consider this one: You seem intelligent. "Intelligent" modifies "you". Would you prefer, "You seem intelligently" ? Go to the rear of the class.

On a side note: When I was in school, a very boneheaded English Grammar text tried to tell us that "I feel well" was correct, rather than "I feel good." The committee of English teachers who wrote this book were wrong. Thanks to fifty years of not teaching grammar in "grammar school", they didn't understand that sentence well enough to parse the connecting verb and realize that "well" was totally inappropriate. They tried to slither around the issue by claiming that "in this instance, 'well' functions as an adjective," this admitting that an adjective was the appropriate choice. I feel sick, I feel happy, I feel inappropriate. Not I feel happily or inappropriately, those would describe what your hands were doing. (Shame on you!)

I remember there was a time when Prager used to say, "Boys and girls are born differently." The grossness of this error should be immediatlly obvious. You can be born in a certain way, but once you are born you have certain characteristics and don't confuse those characteristics with the manner in which you were born. Obviously someone told Prager how wrong this was, but I can't for the life of me figure out why he didn't retain the lesson and generalize it to other copulative verbs.