While I adore a strategically delivered adverbial (word, phrase or conjunction—I bend the knee to the best among them!) —the gods of grammar and good writing discourage their usage because of, ironically, misusage. As the venerable old Crusty, Mr. William Strunk, Jr., demanded in 1918 in his seminal booklet, The Elements of Style, “Omit needless words.” And adverbs got the boot. Adjectives fared a bit better, but not by much.
Years later, the writer E. B. White expanded and revised the book. You’ll hear the short-hand reference to “Strunk and White” since then, said with the same kind of nasal intonation as an aging tennis player might say “my Tretorns, pal” or a millennial hipster might not say anything aloud (probably won’t) but his feet, in perfect California Vowel Shift, say “my Nikes, bruh” loud and clear!
To understand what is wrong with the phrase you mentioned, one must have a passing familiarity with quantum physics, the T. S. Eliot poem, “The Wasteland” and how the phenomena of lesbianism has gone from “Oh, pshaw, they don’t exist” to “My Gawd, they’re everywhere!” As I have no way of knowing if you are or know any physicists, poets or lesbians, allow me to clarify.
If you were to say (in the ACT example) “In addition, LEDs last extremely longer than standard light bulbs,” you would be in violation of English Language Rule, XYZABC.9875642583451, which states: Never use the adverb “extremely” to modify the adjective “longer.” Well, all right, there isn’t actually such a rule. So, new? (Say it with a Noo Yawk accent, it sounds better!)
Well, and that’s the other thing about adverbials, isn’t it! Rules. Far too many rules. Extremely unfortunate too!
What began as a simple, even aesthetic, preference for modifying verbs and others with a lovingly hand-picked, fretted over adverb, the lowly “-ly” words, the poor fifth cousins of the eight parts of speech family, headed by, don’tcha know, those Nouns, well, the entire affair leaves a bitter taste. It is as if the least among us have committed a veritable Crime Against Style.
Not surprisingly (because: War of Independence!), most American English grammar rules are, more or less, the same as those proposed by the British Council, although we differ on spellings rather boisterously. And that’s before Brexit! It could have gone to Hades by now for all we know.
The British Council says (with examples, so I won’t repeat them here): “We use adverbs to give more information about the verb.”
The Council goes on to say there are adverbs of manner, place, time, and probability. We normally put adverbials after the verb; adverbials, however, may also go after the object; on the other hand, adverbs of frequency go in front of the verb—but, and isn’t there always a “but” in the English language?—if one wants to emphasize an adverbial, we can put it at the beginning of a clause; furthermore, if we want to emphasize an adverb of manner, we’re allowed to put it in front of the main verb.
But perhaps the single best argument against saying “extremely longer” is that it elucidates nothing. That “omit needless words” thing, again! For that matter, “far longer” elucidates even less. Why not merely say the LEDs last longer? To know how much longer LEDs last compared to conventional light bulbs, we have to first know how long conventional light bulbs last. It’s not as if the Board of the ACTs doesn’t know this stuff!
Therefore, notwithstanding the ACT tests on which I personally did not score far higher or even extremely higher than my peers, I think the ACT question is unhelpful at best and possibly extremely unfair. Thus, I feel your pain, if you had any, that is.
I do not dare to hope this explanation has been helpful, but in matters of the heart, whiskey and English language grammar, one is often happy enough to be entertained.