*Farly – the loss of adjectival adverbializing suffixes
Far is more often an adverb than an adjective (throw it far vs. the far hills) and “hard” is easy to find as an adjective, but in work hard, it is clearly an adverb, one of the ten or so monosyllabic Germanic ones which have lost the possibility of being followed by -ly.
Spell it wrong is for real people, “spell it wrongly” is for pedants. NOBODY says “spell it rightly.”
“Talk soft” sounds a bit hicky to me still, but I’m sure it will be the President’s English before long. “Dress warm” will take longer.
Slow(ly) are both possible, though when I was growing up, mavens were inveighing against things like “drive slow.”
Somebody should write an (incredibly interesting) thesis on this problem, because the loss of adverbial -ly is something that happens in Spanish and Portuguese for rapido “fast”: (vai comer rapido/rapidamente//lentamente/*lento/ devagar(*mente) = Go eat quick//slowly). I think there may be others in Brasilian, but I can’t think of any now. Yes I can! Bem means “well,” (“in a good manner”), and ***bemmente is a laugher. “pessimo” means “really bad,” and is an adjective in uma torta pessima [= a really awful pie], but it can appear with or without -mente: O Haroldo toca pessimo/pessimamente “H plays badly. I think in Maira pulou alto = M jumped high, altamente is impossible. Interestingly, altamente, like “highly,” means only “extremely,” not something like “extremely vertically” or “way uply”
In French, they have vite(*ment) “fast” [une auto vite “a fast car”/ Jean court vite “Jack runs fast.”] – I don’t know about any others.
In German, the process has gone to the max: there is no adverbializing suffix that would correspond to -ly. Goethe schrieb schnell(***lich) = Goethe wrote fast.