Interrupters are modifiers or appositives that comment upon a noun. We enclose them in commas because they are additional information not essential to the meaning of the main clause. The modifiers come in a number of different forms:
Adjective Clauses: Sally, whom you met at last night's dance, wants to know if you found her purse.
Noun appositives look like this:
Jim, sheriff of Monro County, wanted to question the witness.
Noun appositives are noun phrases that rename a noun.
The trick with these phrases and clauses is that they should be enclosed in commas only if they are non-restrictive or non-essential. This means only if they are not vital to the meaning of the sentence. If the phrase and claused helps to single out one of many possibilities, then it is vital to the meaning of the sentence:
Essential: The girl whom you met at the dance last night wants to know if you found her purse.
Test makers will make it very clear whether a clause or phrase is essential or non-essential because often the punctuation determines the meaning:
#1: Boys who act like jerks never win.
One of these sentences claims that all boys are jerk. The other asserts that only jerky boys don't win. Do you know which is which? Well, #2 makes the all-encompassing claim that all boys are jerks.
A number of other types of expressions also interrupt the flow of main clauses:
Transitional Expressions: He must, of course, pay his fine. He will, however, serve his time. We have, therefore, been vindicated.
Interrupters that must be set off by commas are those which are to the meaning of the main clause.
Phrases and clauses that single out one among many are and do not have commas around them.