Commas and introductory dependent clauses
A complex sentence contains one independent clause and one or more dependent clauses.
After, although, as, because, before, if, since, unless, when, and while are among the words most frequently used to introduce dependent clauses.
When a dependent clause precedes the independent clause, separate the clauses with a comma. (Dependent clauses in example are italicized.)
Before we can make a decision, we need to review all the prior records.
When you read the mammogram, pay special attention to the left upper quadrant of the right breast.
If the patient has been worked up thoroughly, we can proceed with surgery.
When two dependent clauses both modify the main clause that follows, do not use a comma to separate the dependent clauses.
After she was admitted to the Emergency Room and before she was examined by the doctor on call, the patient had to be restrained.
Commas are not needed after ordinary introductory adverbs or short introductory phrases that answer such questions as the following:
- When: tomorrow, yesterday, recently, early next week, in the morning, soon
- How often: occasionally, often, frequently, once in awhile
- Where: here, in this case, at the meeting
- Why: for that reason, because of this situation
Occasionally the patient feels chest pain when climbing stairs.
Yesterday the patient began to feel chest pain when climbing stairs.
Recently he began having chest pain when climbing stairs.
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