Commas group words together, so that sentences are logically constructed. In some instances, however, sentences read perfectly well without commas setting off groups of words. The practice of omitting commas when appropriate is known as an open punctuation style (as opposed to a closed style, where groups of words are closed off by commas).
One instance in which the comma can be omitted is after short—I repeat: short—introductory adverbial phrases. An adverbial phrase modifies a verb, indicating time, place, manner, or degree. Short introductory adverbial phrases do not require commas when the meaning is clear:
- At night I work on my manuscript.
- Under the chair you will find the cat dozing.
- Little by little she made progress on her goals.
However, a comma is required after a sentence adverb, which modifies a whole sentence (see what I did at the beginning of this sentence?):
- Typically, my colleague forgets his wallet when we go to lunch.
- Unfortunately, I forgot my wallet, too.
(Note: However is technically a transitional adverb, but the rule is the same.)