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)
These adverbs are placed at the beginning of the sentence and are separated from the rest of the sentence by a comma.
Some common Viewpoint adverbs:
honestly, seriously, confidentially, personally, surprisingly, ideally, economically, officially, obviously, clearly, surely, undoubtedly.
Personally, I'd rather go by train.
Surprisingly, this car is cheaper than the smaller model.
She is certainly the best person for the job.
You obviously enjoyed your meal.
These are very similar to viewpoint adverbs, and often the same words, but they go in a different position - after the verb to be and before the main verb.
Some common Commenting adverbs:
definitely, certainly, obviously, simply.