Halliday & Matthiessen (2014: 419):
The adverbial group has an adverb as Head, which may or may not be accompanied by modifying elements. Adverbial groups serving as circumstantial Adjunct have an adverb denoting a circumstance as Head – for example, a circumstance of time (e.g. yesterday, today, tomorrow) or of quality (e.g. well, badly; fast, quickly, slowly). Adverbial groups serving as modal Adjunct have an adverb denoting an assessment as Head – for example, an assessment of time (e.g. still, yet, already) or of intensity (e.g. really, just, only, actually). Examples are given in Table 6-18. As the table shows, some classes of adverb have interrogative and demonstrative forms.