Introductory clauses are dependent clauses that are often found at the beginning of the sentence (although they can be moved to the end of the sentence, too, without confusing the meaning of the sentence). After a dependent introductory clause, we use a comma to separate the introductory clause from the independent clause. Consider the examples below:
As with many other rules in English grammar, the comma-after-introductory-clauses rule comes with an exception. When the introductory clause is short, the comma may be omitted as a matter of style. See the example below:
Since the introductory clause consists of only three words, the comma separating the introductory clause from the main clause may or may not be used.
Introductory Clauses with Dates
Although it is not strictly required, it is considered good style to follow introductory dependent clauses containing dates with a comma. This is true whether the date given denotes a century, a year, a month, or even a day.
But if the date does not appear as an introductory clause, no commas are necessary.