1. Wrong: If he trains his dog to joggle (jog and juggle simultaneously) he could make a lot of money.
Better: If he trains his dog to joggle (jog and juggle simultaneously), he could make a lot of money.
Explanation: "If he trains his dog to joggle" is an introductory adverbial clause.
2. Correct: Craig would love to sing in the musical if he gets a starring role.
Explanation: Because the adverbial clause comes at the end of the sentence, it does not need to be separated with commas.
3. Correct: Buying a new car can cause stress when money becomes tight.
Explanation: Because the adverbial clause follows the independent clause, it doesn't need to be preceded with a comma.
4. Wrong: Because she understood fractals she aced the exam.
Better: Because she understood fractals, she aced the exam.
Explanation: The first four words make an adverbial clause, so they need to be separated with a comma.