Adverbs modify verbs, adjectives or other adverbs, but a special category can modify an entire sentence. Properly called sentential adverbials, these words or phrases are sometimes controversial in grammar circles, because sentences can function structurally without them. Used conscientiously in literature, however, they can subtly alter a sentence’s meaning, change its emphasis or improve its rhythm.
For Meaning
Some sentential adverbs can alter the meaning of a sentence, if subtly. In “L’Ingenu,” for instance, Voltaire writes, “In fact, history is nothing more than a picture of crimes and misfortunes.” The adverb phrase “in fact” underscores the irony of the Huron’s seeking greater understanding, but finding only melancholy.
For Emphasis
In other cases, the sentential adverb does not change the meaning, but the emphasis. Consider the famous line from the film “Gone with the Wind”: “Frankly, my dear, I don’t give a damn.” In Margaret Mitchell’s novel, Rhett Butler does not speak the sentential adverb “frankly,” and although the meaning is the same, the line lacks the screenplay's verbal punch.
For Rhythm
Because sentential adverbs are so common in spoken English, writers often insert them for rhythmic effect, rather than to affect the meaning or emphasis. In the poem, “Keeping Track of My Genius,” Jack Stewart uses the sentential adverbs “usually” “of course” and “for instance" in this way.