Introductory words and introductory sentences are used to make the text more harmonious. These are original words-assistants, which establish a logical connection between parts of the utterance. Syntactically they are not related to other words, that is, they can not be asked a question, they are not members of the sentence and they lose morphological features, but their role is very important for understanding the meaning of the text. They accurately convey how the speaker relates to the main content of the utterance.
Introductory words are divided into special thematic groups. Each of these groups performs its function. For example, introductory words that express the speaker’s feelings (fortunately, to sorrow, which is good) give the text emotionality. The words indicating the source of the message (in my opinion, according to scientists, according to rumors), make the statement subjective or, conversely, objective. And the words that carry out the connection of thoughts, the sequence of presentation (so, first of all,) and pointing to the devices and ways of formulating thoughts (in a word, in other words, so to speak) serve as means of communication between parts of the sentence, sentences and even microtemes.
In the role of opening words, independent parts of speech are often used. To check if the word is introductory, you need to remember that these words are easily removed from the text without sacrificing meaning. For an independent part of speech, this is impossible.
Introductory words help to more accurately convey their feelings and thoughts, make the statement more accessible and expressive. And the reader, they give the opportunity to immediately understand the written and read the text with the desired intonation.