§ 244.Modal words express the speaker’'s attitude to what his utterance denotes. The speaker’s judgement may be of different kinds, that is, the speaker may express various modal meanings.
Modal words are an invariable part of speech. They may refer to a word, a phrase, a clause, or a sentence. Their syntactical function is that of a parenthesis, they may also be a sentence in themselves, in which case they are used to answer a general question:
Will you help me? Certainly.
Precisely this.
Except this man, of course.
Semantically modal words fall into three groups, denoting:
1.Certainty/doubt(certainly, of course, indeed, surely, decidedly, really, definitely, naturally, no doubt,
etc.).
Certainly, it was astonishing that she should be preoccupied with her schemes for the welfare of
Constance.
Of course, it would have been different if they had married.
In answers the meaning of these words is weakened.