Final answer:
Introductory words and phrases require a comma afterwards, nonessential information within a sentence is set off by commas, and dependent clauses preceding independent ones typically entail the use of a comma unless no contrast is implied.
Step-by-step explanation:
Introductory words and phrases play an essential role in writing, as they help to create flow and continuity between sentences, leading to clear and cohesive expression of ideas. When starting a sentence with an introductory element such as a word, phrase, or clause, it is standard practice to follow it with a comma.
This technique indicates to the reader that the main part of the sentence is about to begin. One common example is the use of transitional words - like 'however,' 'therefore,' or 'meanwhile' - which, when used at the beginning of a sentence, should be followed by a comma.
Understanding the difference between nonessential and essential information is crucial when applying commas. Nonessential information, which can be omitted without altering the basic meaning of a sentence, should be isolated by commas.
Conversely, essential information, critical to the core meaning of a sentence, should not be enclosed by commas. For example, 'The labs, which were outdated, needed to be renovated.'
Another key point is the use of commas in compound and complex sentences. A comma must accompany a coordinating conjunction when connecting two independent clauses.
Similarly, when an opening dependent clause prefaces an independent clause and is introduced by subordinating conjunctions like 'if' or 'when,' a comma is needed following the dependent clause. However, if the dependent clause follows the independent clause and there is no contrast implied, a comma is not necessary.