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?)
1 and 4 are correct.
Since "well" is an adverb modifying the very "is known" or "are known," there is no hyphen needed.
To determine which options are grammatically correct, we need to understand the subject-verb agreement.
In sentence 1, "waterfall" is singular, so it should be accompanied by a singular verb. Therefore, "is" is correct. Thus, option 1 is grammatically correct.
In sentence 2, "fall" is also singular, so it should also be accompanied by a singular verb. Again, "is" is correct. Hence, option 2 is grammatically correct.
In sentence 3, "falls" is plural, so it should be accompanied by a plural verb. In this case, "is" is not appropriate because it's singular. Instead, "are" should be used to maintain subject-verb agreement. Therefore, option 3 is not grammatically correct.
In sentence 4, "falls" is plural, and it is correctly paired with the plural verb "are." Hence, option 4 is grammatically correct.
Therefore, options 1, 2, and 4 are grammatically correct choices.