In the CGEL(Quirk et al), the case below puzzles me because an adverb phrase or a prepositional phrase rarely serves as subject.
10.15 Adverbial forms as subject:
(1) Slowly is exactly how he speaks.
(2) Will after the snow be soon enough?
It is said in the book that the following two conditions allow this use of adverbials.
(i)the adverbial is a fragment of understood clause(of 15.16), or
(ii) the sentence can be related to one with prop it(of 10.26)
I've read extremely carefully on every detailed term either of 15.16 or of 10.26. Condition (ii) is understandable, whereas Condition (i) is way too difficult to understand.
How can I judge whether the adverbial is a fragment of understood clause?
How does a non-native speaker define the understood clause?