it is a common practice to separate an adverb at the beginning of a sentence from the rest with a comma. However, I have read somewhere that we can omit the comma when no pause is needed. Is this correct?
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When a single adverb -- I'm not going to get into adverbial phrases because you didn't ask about them -- introduces a sentence, appearing in advance of the subject and verb of the main clause, the rule is to follow it with a comma. The exception to this rule is when the adverb is also being used somewhat conjunctively.
- John went to the store then he went home.
In the above, "then" is being used as a subordinating conjunction that introduces the subordinate clause "he went home." When "then" starts a sentence, however, it's not a conjunction but an adverb, so you will often see a comma after it as follows:
- John went to the store. Then, he went home.
However, oftentimes, we hear people rattle off that second sentence without even the tiniest hesitation between "then" and "he." When people do that, it's been construed that the adverb "then" is borrowing on its subordinating conjunction definition, so it has become acceptable to write it as follows:
- John went to the store. Then he went home.
Owing to how the adverb "then" appears without a comma after it, we also hear people using "now" in the same way, but "now" isn't a subordinating conjunction. "Now" is only an adverb. Nevertheless, it has become acceptable to write it without a comma when it seems to be used more as a conjunction to append a new sentence to a prior sentence, for example:
- I lost my keys. Now I don't know what to do.
You should know, though, that not putting a comma after an introductory adverb, whatever the adverb, is somewhat controversial grammatically. If you want to never be wrong, always put a comma. No one can fault you for putting a comma after an introductory adverb, but you may be faulted for not doing so.
More standard adverbs, adverbs that aren't used conjunctively, adverbs that end in "-ly" for example, appear with a comma after them nearly without exception.