Here's why:
* Conjunctions connect words, phrases, or clauses of equal grammatical rank.
* Subordinating conjunctions introduce dependent clauses (clauses that cannot stand alone) and connect them to independent clauses.
Example:
* When the sun sets, the birds start to sing.
* "When the sun sets" is a dependent clause introduced by the subordinating conjunction "when" and is connected to the independent clause "the birds start to sing."
While "when" can sometimes be used to connect two independent clauses, it's more accurate to consider it as a conjunction in those cases:
* "I was studying, when suddenly the lights went out."
* This is a case where "when" acts like a coordinating conjunction, connecting two independent clauses. However, it's less common usage than as a subordinating conjunction.
In summary, "when" is primarily a subordinating conjunction used to introduce dependent clauses. It can sometimes act as a coordinating conjunction, but this is less common.