dependent clauses: when to use a comma
The comma with a dependent clause first or last.
When a dependent clause comes first, follow it with a comma: If the train is late, wait in the hall. When the dependent clause comes last and is restrictive, use no comma: Wait in the hall if the train is late. When the dependent clause comes last and is nonrestrictive, set it off with a comma: We took the side door, although the hall was clear.
Examples
UseWhen the bell rings, stand.
UseStand when the bell rings.
UseWe took the side door, although the hall was clear.
AvoidWhen the bell rings stand.