Answer:
1. If it rains, we won't go to the beach.
2. Ms. Smith, who lives next door, came to borrow some sugar.
3. You must be really proud of your mother, aren't you?
Explanation:
1. The use of the comma is obligatory after introductory clauses or phrases. All the markers for the introductory clauses are always followed by a comma (if, since, while, after, because, etc.).
2. The use of the comma to offset a clause in the middle of a complete sentence is mandatory, as those clauses provide additional information to the complete sentence.
3. The use of comma to offset a clause at the end of a sentence is mandatory, in order to separate contrasting elements or to denote a pause during the speech.
