Here's why:
* Connects clauses: It joins a dependent clause (the "if" clause) to an independent clause.
* Shows a relationship: The "if" clause usually expresses a condition, and the independent clause states the consequence of that condition.
Example:
* If you study hard, you will pass the test.
* "If you study hard" is the dependent clause, and "you will pass the test" is the independent clause. "If" connects these clauses and shows that passing the test is dependent on studying hard.