Which is correct?
Some human engineering efforts are so blatant that they fool only the ________ end users.
a. most stupid
The rules are clear on how to form comparatives and superlatives — as long as the adjective or adverb you’re trying to modify has fewer or more than two syllables. For two-syllable words, like “stupid,” the rules get a little murky and sometimes you’re just going to have to rely upon your ear or take your best guess.
The general rule is: For the comparative/superlative forms of adjectives and adverbs, add er/est to one-syllable words and plunk more/most (or less/least) in front of words of two or more syllables. But there are exceptions to every rule and this one has more exceptions than most.
On Grammar Girl, Bonnie Trenga wrote about deciding on whether to add er/est suffixes or add more/most to form the comparative/superlative forms of adjectives. She found a rule for the exceptions to the rule:
I did find one rule to help guide you: two-syllable adjectives that end in “-y,” “-ow,” and “-le” can take the suffixes “-er” and “-est” (2). Remember that by thinking they’re y-ow-le howl-ey! Y-ow-le. Or better yet, think that they are yowlier and howlier than everything else, so you remember the adjective endings “-y,” “-ow,” and “-le,” and the rule to end them with “-er” or “-est.”
But that still leaves us with “most stupid” as the answer, which just doesn’t sound right. I’m with “stupidest.”
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