Adverbial. I shall see you when the sun next rises. (tomorrow, adverb)
Now there is no rule that subordinate clauses must be separated from the main sentence by a stop; that depends on whether they are essential parts of the proposition (when stops are generally wrong), or more or less separable accidents (when commas are more or less required). But what we wish to draw attention to is a distinction in this respect, very generally disregarded, between the substantival clause and the two other kinds. When the others are omitted, though the desired meaning may be spoilt, the grammar generally remains uninjured; a complete, though not perhaps valuable sentence is left. The man is respected, I shall see you, are as much sentences alone as they were with the adjectival and adverbial clauses. With substantival clauses this is seldom true; they are usually the subjects, objects, or complements, of the verbs, that is, are grammatically essential. He asked is meaningless by itself. (Even if the point is that he asked and did not answer, things, or something, has to be supplied in thought.) Now it is a principle, not without exceptions, but generally sound, that the subject, object, or complement, is not to be separated from its verb even by a comma (though two commas belonging to an inserted parenthetic clause or phrase or word may intervene). It follows that there is no logical or grammatical justification, though there may be a rhetorical one, for the comma so frequently placed before the that of an indirect statement. Our own opinion (which is, however, contrary to the practice of most compositors) is that this should always be omitted except when the writer has a very distinct reason for producing rhetorical impressiveness by an unusual pause. Some very ugly overstopping would thus be avoided.
Yet there, too, we find, that character has its problems to solve.–Meredith.
We know, that, in the individual man, consciousness grows.–Huxley.
And it is said, that, on a visitor once asking to see his library, Descartes led him...–Huxley.
