subordinate clauses: when to use a comma

A subordinate clause has its own subject and verb but begins with a subordinating conjunction, such as if, when, because, or although, that keeps it from standing alone as a sentence. Whether it takes a comma depends on where it falls.

A subordinate clause that opens a sentence takes a comma after it: When the power came back, the alarm clocks all started flashing.

A subordinate clause that interrupts the main clause takes a comma on each side: The ceremony, unless it rains, will be held outdoors.

A subordinate clause at the end of a sentence takes no comma unless it is parenthetical, states a contrast, or needs the comma to prevent a misreading: The kitchen closes when the last order is served. A clause that states a contrast, such as one beginning with although, though, or whereas, takes a comma: The kitchen stayed open, although the dining room had emptied.

After a negative main clause, a comma before because can decide the meaning. Tom didn’t call the client because he was angry can mean that Tom called the client for some other reason. Tom didn’t call the client, because he was angry means that Tom did not call, and that anger was the reason.

See also comma with introductory elements; dependent clause / independent clause; subordinate conjunction / preposition; comma with nonrestrictive clauses; comma.