USING SUBORDINATE CLAUSES
WHO (also WHOEVER) is used for subjects; WHOM (also WHOMEVER) is used for objects. In subordinate clauses, look to see how they are used in the clause they are in.
I know who committed the crime. (Subject of the verb COMMITTED in the subordinate clause.)
Jack noticed whom you were talking to. (Object of the preposition TO. Rephrase: You were talking to WHOM.)
COMMAS AFTER NONESSENTIAL INFORMATION
In subordinate clauses, commas are used to separate additional information about something.
Bill, who called earlier, will call back.
The red Buick, which my uncle has restored, is an antique.
Here we can identify the person and the car without the information in the subordinate clause. Test it by eliminating the subordinate clause. If you know what is being discussed without the clause, use commas to enclose the subordinate clause.
ESSENTIAL INFORMATION (RESTRICTIVE CLAUSES)
No commas are used when the subordinate clause is necessary to identify what is being discussed.
The man who called earlier will call back.
The car that my uncle restored looks terrific.
Here we can't identify which man or which car is meant without the information in the subordinate clause, so no commas are used.
INTRODUCTORY SUBORDINATE CLAUSES
When a sentence starts off with a subordinate clause, use a comma after it unless it's the subject of the main clause. When we go to the movies, we have a good time.
Because I was late, I missed my plane.
However, when the subordinate clause is the subject of the main clause, do not use a comma:
What you did is inexcusable.
That he cheated is clear.
SUBORDINATE CLAUSES AS MODIFIERS
Put subordinate clauses near the words they modify.
The record that I bought yesterday is defective.
The window was broken by a rock that Jenny threw.
Nouns or subordinate clauses that follow a noun and rename it, called appositives, are set off by commas. Jesse refused to answer all questions, whatever they were.
The contestants had to accept the judges, whoever they were.
OMITTING THE SECOND THAT
When two subordinate clauses are used together, the second THAT is omitted.
WRONG: I know that if you try that you'll succeed.
RIGHT: I know that if you try you'll succeed.