Abstract
The group of expressions in question are words homonymous with qualitative adverbs (určitě, nepochybně, vážně). Their semantics, though, is not related only to a verb/predicate as its qualitative modifier; instead, it is related to the whole sentence/proposition (hence the term sentence adverbials) as an expression of speaker´s epistemic or evaluative attitude towards the proposition, cf. mluvil vážně/he spoke seriously vs. vážně mluvil dobře/seriously, he spoke well.
In the attitudinal function, they are often subcategorized among particles. In teaching practice, the difference between those two functions can be shown as an im/possibility to replace the attitudinal expressions by lexically related adjectives in nominalized constructions: jeho vážná (pro)mluva/his serious speech vs. jeho vážně dobrá (pro)mluva/his really serious speech.
Adverbials with the meaning of sensoric perception (očividně/visibly, slyšitelně/audibly, hmatatelně/palpably) or hearsay (údajně/allegedly) represent a specific group. Even though they can be repalced by derivationally related adverbs, their core function is to express speaker´s evidential attitude, i.e. their function is primarily pragmatic.