The grammar of Kühner & Stegmann (K.–St. I. 793) distinguishes as a subclass of the class of adverbs those adverbs which express the judgment of the speaker with regard to the content of the sentence, terming them `Modaladverbien'. Within this subclass, a further subdivision is made between adverbs which
– confirm: e.g. sane (`certainly'), vero (`truly')
– deny: e.g. non (`not'), haud (`not (at all)')
– reinforce: e.g. profecto (`indeed'), certe (`surely'), scilicet (`evidently')
– indicate uncertainly: e.g. fortasse (`perhaps')
– mark a question: e.g. num (`surely not … ?'), utrum (`whether')These groups are dealt with individually in K.–St. I. 795 ff. In reality, they contain a very heterogeneous collection of expressions, which are, moreover, certainly not in all cases `sentence adverbials': some of them, for instance, may be used independently, e.g. as an answer to a question (cf. fortasse in (1), certe in (2)), whereas interrogative particles cannot occur independently (they do not have `sentence valency').
(1) frugi tamen sum … :: fortasse (`Yet, I am virtuous :: Perhaps', Pl. As. 499)
(2) miser ergo Archelaus? :: certe, si iniustus (`So A. is miserable? :: Certainly, if he is unjust', Cic. Tusc. 5.35)In recent studies the term `sentence adverbials' is limited to those adverbs which can indeed be used as an answer to a neutral question (a yes–no question). It is precisely on this point that they are distinguished from `normal adverbs', which only occur as an answer to certain question words, such as `when', `where', and the like. Sentence adverbials in the current sense of the term are furthermore characterized by their inability to occur in all sentence types (interrogative, imperative and declarative sentences). [Note] A number of the adverbs mentioned by K.–St., viz. those expressing reinforcement and uncertainty (e.g. haud dubie (`doubtless'), scilicet (`evidently'), fortasse (`perhaps'), nimirum (`surely')), are not found in imperative sentences, cf. crosssection 7.1.2. In the light of these criteria, the behaviour of the group sane, vero, haud and non (confirmation/denial) differs from that of the first group of words. 
Pinkster, Harm (1942-) , Latin Syntax and Semantics [info], xii, 320 p.: ill.; 24 cm. [word count] [Pinkster].