Question

1. The waterfall is well-known.
2. The fall is well-known.
3. The falls is well-known.
4. The falls are well-known.

(Which ones are grammatically correct?)

Answer 1

1 and 4 are correct.

Answer 2

Since "well" is an adverb modifying the very "is known" or "are known," there is no hyphen needed.