Is "drive slow" grammatically correct?

Yes. "Slow" is a legitimate flat adverb that has been used by writers like Shakespeare and Milton for centuries. While "slowly" is the preferred form for formal essays, "drive slow" is grammatically sound, especially in short commands and on road signs.

Why is it "sleep tight" and not "sleep tightly"?

"Sleep tight" is an idiomatic expression that uses a flat adverb. In this context, "tight" means securely or soundly. While you can "fasten a bolt tightly," the phrase "sleep tight" has become a fixed idiom in the English language where the -ly version would actually sound unnatural.

What is the difference between "real" and "really" as adverbs?

In formal writing, you should always use "really" to modify an adjective (e.g., "really good"). However, in casual American speech, "real" is frequently used as a flat adverb (e.g., "that’s real nice"). While common in dialogue, using "real" as an adverb is often flagged as an error in professional or academic settings.

Is dropping the -ly from adverbs a new trend?

Actually, it is the opposite. Flat adverbs were the standard in Old English. The push to add -ly to every adverb only gained momentum in the 18th century when grammarians tried to make English rules more like Latin. Using flat adverbs is a return to the language's original roots.

Are flat adverbs a sign of "bad grammar"?

Not necessarily. Many of the most common adverbs in English have no -ly form at all, such as "fast," "hard," and "late." No one would say "he runs fastly." However, using flat forms for words like "quick" or "bright" in a formal research paper may be viewed as too informal by some readers.

Can flat adverbs modify other adverbs?

Yes, though this is primarily found in informal dialects. You might hear phrases like "that happened mighty quick" or "he ran crazy fast." In these cases, "mighty" and "crazy" act as flat adverbs modifying other adverbs.