Comment and viewpoint adverbs add information about the speaker’s opinion of events. They do not usually give information about how something happened. Comment and viewpoint adverbs often modify the complete sentence, not just the verb .
Geoffrey Leech in his Communicative grammar of English marks them out as ‘sentence adverbials’ [5, 181]. He says that they are peripheral to the sentence structure. That’s why it’s important to differentiate comment and viewpoint adverbs from other groups of adverbs.
Compare these sentences:
- She started singing happily. (adverb of manner)
- Happily, she started singing. (comment adverb)
In sentence 1, the adverb – happily – gives more information about how she was singing. In sentence 2, the adverb – happily – gives the speaker’s comment/opinion of the event. In this case, the speaker thinks that her starting to sing was a happy event .
There is a list of viewpoint and commenting adverbs below:
Bravely, Carelessly, Certainly, Clearly, Confidentially, Definitely, Disappointingly, Economically, Foolishly, Fortunately, Generously, Happily, Honestly, Ideally, Kindly, Luckily, Naturally, Obviously, Officially, Personally, Presumably, Rightly, Seriously, Simply, Stupidly, Surely, Surprisingly, Technically, Theoretically, Thoughtfully, Truthfully, Unbelievably, Undoubtedly, Unfortunately, Unluckily, Wisely, Wrongly
Frankly, I think he is a liar. (= this is my frank, honest opinion)
Theoretically, you should pay a fine. (= from a theoretical point of view but there may be another way of looking at the situation)
Confidentially, Gregory viewed his move from Zoology to Anthropology as a quiet, secret “sort of revolt” [COCA, NEWS, 2011].
Now that Brett was 16, kids his age weren’t playing cops and criminals anymore. (Stupidly, a couple had already become real criminals by way of shoplifting and trespassing.) [COCA, FIC, 2010].
Truthfully, there aren’t many people who could look like this man [COCA, SPOK, 2009].
Commenting adverbs are very similar to viewpoint adverbs, and often the same words. Raymond Murphy gives next classification of commenting adverbs :
Adverbs indicating thinking something:
Apparently, Certainly, Clearly, Definitely, In theory, Obviously, Presumably, Probably, Undoubtedly
Apparently, the participants held positive attitudes toward the English language program they follow [COCA, ACAD, 2011].
Certainly, we see evidence of this impulse in the animal kingdom; a small bird will dive onto a hawk to protect its nest [COCA, NEWS, 2011].
Presumably, their wives, daughters, and sisters watched the game at home, in the company of other women [COCA, ACAD, 2010].
Adverbs indicating the attitude to or opinion of what is said:
Astonishingly, Frankly, Generally, Honestly, Interestingly (enough), Luckily, Naturally, In my opinion, Personally, Sadly, Seriously, Surprisingly, Unbelievably
There are some examples:
Luckily, the situation is unlikely to be serious [COCA, MAG, 2011].
Astonishingly, Apple has finally given its blessing to running Windows on a Mac, with this utility [COCA, MAG, 2006].
Unbelievably, he’s running as fast as the bus [COCA, FIC, 2006].
Adverbs showing judgement of someone’s actions:
Bravely, Carelessly, Foolishly, Generously, Kindly, Rightly, Stupidly, Wisely, Wrongly
Wisely, Daria refuses to have a fax machine in her apartment or else all her insecure, praise hungry authors would be faxing her their books page by page then demanding an hour’s praise for every paragraph that they hope is wittily written [COCA, FIC, 1997].
Carelessly, a young man takes the seat across from me [COCA, FIC, 1999].
Unbelievably, it was the sound of gunfire captured on a tourist’s home video [COCA, SPOK, 1997].
There are other possible positions for each of the comment adverbs in these examples. To show that they apply to the whole sentence, they are usually separated from the rest of the sentence, particularly in front and end positions, by a comma in writing or by intonation in speech. A number of phrases and clauses can be used in a similar way to comment adverbs to indicate the attitude to, or opinion of, what is said. For example:
Oddly enough, she didn’t mention that she was moving house. (Also Curiously/Funnily/Strangely enough) 
Among the group of commenting and viewpoint adverbs, evaluative and illocutionary adverbs should be also considered.
Evaluative adverbs can be selected into the subdivision:
Conveniently, Curiously, Fortunately, Happily, Interestingly, Ironically, Luckily, Oddly, Paradoxically, Regrettably, Sadly, Surprisingly, Unfortunately
Quirk et al., however, distinguish two sub-groups under the general group of adverbs conveying “value judgment” . The adverbs of the first sub-group “express a judgment on what is being said as a whole and they normally apply the same judgment simultaneously to the subject of the clause” . The adverbs of the second sub-group express a “judgment that carries no implication that it applies to the subject of the clause” . Adverbs such as foolishly and rightly fall in the first sub-group while the adverbs listed above belong to the second sub-group.
Huddleston and Pullum identify a group of “evaluative clause-oriented adjuncts” [15, 675], under which the adverbs curiously, disappointingly, fortunately, funnily, happily, luckily, oddly, regrettably, sadly, strangely, surprisingly, and unfortunately fall. The authors do not consider, however, adverbs such as foolishly, rightly, and stupidly to be part of that group. Instead, they classify these adverbs under another group that they call “act-related adjuncts”, which is oriented to the verb phrase and not to the whole clause. Adverbs like accidentally, knowingly, and reluctantly belong to this latter group. In addition, Huddleston and Pullum distinguish two sub-types within the group of “act-related adjuncts”. The first sub-type includes adverbs like cleverly, foolishly, rudely, and wisely, while the second sub-type includes adverbs like accidentally, deliberately, intentionally, knowingly, and willingly. The adverbs that fall under the first sub-type of the “act-related adjuncts” in Huddleston and Pullum’s classification involve not only an evaluation of the act by the speaker but also an evaluation of the actor too. While such adverbs may occupy initial position in a sentence and may be relatively flexible in occupying other positions, they cannot be considered as qualifying the whole utterance. Consider the following examples [15, 675-676]:
(1) He answered the question foolishly.
(2) He foolishly answered the question.
(3) Foolishly, he answered the question.
The same form of the adverb foolishly complements the verb in (1), while it qualifies the verb in both (2) and (3). Note that only (1) can be given as an answer to the question „How did he answer the question?” In none of the above cases, however, can the adverb be considered as qualifying the whole utterance, not even in (3) where the adverb appears in front position.
The other sub-group that Huddleston and Pullum distinguish within the group of “act-related adjuncts” includes adverbs such as accidentally, deliberately, and willingly, which “do not reflect a subjective evaluation of the act but relate to the intentions or willingness of the agent” [15, 676]. Moreover, they are less flexible in the positions they can occupy in the sentence. Nevertheless, none of the sub-groups of “act-related adjuncts” can be said to qualify the standpoint that can be reconstructed from the utterance in which they appear. This is because the comment that they add pertains to the agent involved in the event that is described and not to the event as a whole.
Thus, the following adverbs are filtered out from the list of the stance adverbs that can be used to qualify a viewpoint evaluatively: accidentally, amiably, aptly, carelessly, cautiously, cleverly, consciously, cunningly, deliberately, effectively, foolishly, harmfully, inappropriately, incorrectly, kindly, mildly, mistakenly, prudently, rightly, selfishly, startlingly, suitably, suspiciously, unjustly, unwisely, wisely.
The adverbs that can be used to qualify a viewpoint in the “illocutionary” way belong to the group of adverbs that are known in the literature as “illocutionary adverbs” , or “speech-act related adverbials” . In this section, besides presenting the adverbs like actually, in fact, in reality, and of course, may also count as qualifying the viewpoint by commenting on the act as a whole.
The so-called “illocutionary adverbs” unlike modal, evidential, domain, and evaluative adverbs, add a comment to the act that the speaker performs by means of uttering the sentence in which they appear. The sentences in which they appear can be paraphrased with an explicit illocutionary verb like tell, admit, order, inform, ask. The adverb in this paraphrase functions as a manner adverb modifying the illocutionary verb and thus describes the way in which the act is being performed :
Frankly, it was a waste of time > I tell you frankly it was a waste of time.
Literally, it was a waste of time > I tell you literally it was a waste of time.
This property of illocutionary adverbs explains why they can occur with performative verbs other than just those indicating an assertive illocutionary point, as the following examples  illustrate:
Sincerely, I apologize for being so rude.
Briefly, I promise you to finish my work.
Precisely, I order you to get out of here.
Of the group of illocutionary adverbs, those that comment on the speaker’s own sincerity, such as frankly, honestly and seriously, can also be used to emphasize the truth of what the speaker says, as the example below illustrates:
I’m so happy for you! Honestly, I’m really happy for you! 
However, this is not possible with the rest of the adverbs that are usually listed in the group, such as briefly, literally, metaphorically, and strictly. As Quirk et al. remark, adverbs like actually, indeed, of course, and really together with honestly, frankly and certainly, clearly, obviously, and surely may also function as „emphasizers”, reinforcing the truth value of the clause.
The adverbs certainly, clearly, obviously, and surely have been dealt with as modal adverbs indicating strong degree of commitment to the truth of the propositional content. The adverbs actually, indeed, of course and really appear, in the literature, either in the same group as epistemic adverbs or in related groups.