§ 473. Parenthetical elements are peculiar parts of the sentence.
a) They are characterized by negative combinability with
the other words of the sentence.
b) They are, as it were, not in a line with the other parts
of the sentence, but parallel to them.
c) They mostly express the speaker's attitude towards the
content of the sentence, its relation to other sentences or si
tuations.
One poem, of course, is much the finest. (Galsworthy). By the way, there's to be a preface. (Ib.).
§ 474. In accordance with their meanings parenthetical elements fall into four major groups:
1. Modal parenthetical elements serving to show the atti
tude of the speaker towards the relation of the communication
to reality.
He would have to buy them out, of course. (Galsworthy). Certainly he thought about it all the way there. (Lewis).
2. Connective parenthetical elements showing the con
nection of thoughts.
He did not, however, neglect to leave certain matters to future consideration. (Galsworthy).