impersonal pronoun "it"
- To exercise regularly is important.
- Exercising regularly is important.
- It is important to exercise regularly.
- It is important exercising regularly.
Is the fourth sentence grammatically correct? Can a gerund be a subject (like the example 4)?
Solution 1:
[1] To exercise regularly is important.
[2] Exercising regularly is important.
[3] It is important to exercise regularly.
[4] It is important exercising regularly.
[1]-[3] are fine. Note that the extraposition construction in [3] has the non-referential dummy pronoun "it" as subject.
[4] as written is not acceptable. Gerund-participial clauses are found extraposed, but only under sharply limited conditions. [4] is in fact a 'right dislocation' construction where, unlike in [3] the later element does provide a referential interpretation/clarification of the pronoun.
I haven’t seen a detailed attempt to say when gerund-participials are OK in extraposition, but certainly some cases sound fine. But I remember that in early work in generative grammar it was said simply that extraposition could not apply to gerund-participials.
For those reasons, I'd say that [4] requires a comma, and a pause in speech after "important". This phonological difference makes it a different construction called 'right dislocation'. Unlike the non-referential pronoun "it" in the extraposition construction in [3], the later element in [4] does provide a referential interpretation/clarification of the pronoun.