words that are not syntactically connected to the sentence and express a relationship of the speaker to the information being communicated, a general appraisal of the information, or an indication of the source of the information or its relationship to the context of the conversation.
Parenthetic words may be used as words isolated from other parts of speech and used only as parenthetic words (“of course,” “for example,” and “apparently”) and as words that have retained active relationships with different parts of speech and can be members of the sentence (“it is possible,” “they say,” and “it seems”). Parenthetic words may contain an emotional evaluation of the information (“fortunately” and “unfortunately”) or an appeal for attention (“you know” and “just imagine”). They may indicate a degree of confidence or credibility (“true” and “obviously”), a source (“they say” and “it is known”), style (“simply”), or a relationship between parts of the information (“by the way” and “finally”).
V. V. RASKIN