Here's why:
* It joins two clauses: It connects a dependent clause (the clause that starts with "unless") to an independent clause.
* It shows a conditional relationship: The dependent clause (the "unless" clause) sets a condition for the independent clause.
Example:
* Unless you study hard, you won't pass the test.
* "Unless you study hard" (dependent clause) sets the condition for "you won't pass the test" (independent clause).
Other subordinating conjunctions include:
* because
* although
* since
* while
* if
* when
* after
* before