The Classification of Words

Parenthetical Elements
§ 473. Parenthetical elements are peculiar parts of the sentence.

1. 
   They are characterized by negative combinability with
   the other words of the sentence.
   
2. 
   They are, as it were, not in a line with the other parts
   of the sentence, but parallel to them.
   
3. 
   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).
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May I say, first, that I have been very much pleased by your analysis of the situation (Lewis)
3. Explanatory parenthetical elements.
He remembered suddenly one night, the first on which he went out to dinner alone — an old Malburian din­ner — the first year of their marriage. (Galsworthy).
4. All other words inserted into the sentence, including
direct address:
Shall I announce him, my lady? (Shaw). Then who manages his business, p r a y^ (Ib.).
§ 475. In a simple sentence parenthetical elements may be expressed by individual words (modal words, adverbs, nouns) and word-combinations of different nature.
Perhaps somebody cleverer than you and Mr. Mangan was at work all the time (Shaw).
In my opinion, what the country needs, first and foremost, is a good, sound, business-like conduct of its affairs. (Lewis).
Is he at home here, so to speak, my lady? (Shaw).
§ 476. In most cases parenthetical elements are connected, in sense with the sentence as a whole, that is why they have no fixed position in the sentence.
/ had a rather good night: in fact, one of the most remarkable nights I have ever passed. (Shaw).
Lady Britomart: What were you drinking, may I ask?
Cusins: A Temperance burgundy, in fact. (Shaw).
If a modal word is connected but with one part of the sen­tence it usually precedes the word it belongs to.
They stayed there for, may be, a year.