There are some adverbs and adverbial expressions which tell us about the speaker's viewpoint or opinion about an action, or make some comment on the action.

Viewpoint

Honestly, I think he is a liar.
Theoretically, you should pay a fine.

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:

surprisingly, ideally, economically, officially, honestly, seriously, confidentially, personally,  obviously, clearly, surely, undoubtedly.

Some Examples:

• Personally, I'd rather go by train.
• Surprisingly, this car is cheaper than the smaller model.
• Geographically, Britain is rather cut off from the rest of Europe.

Commenting

• 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.