We use since as a subordinating conjunction to introduce a subordinate clause.

Is since a subordinate conjunction?

Other subordinating conjunctions that can show cause-and-effect relationships and function in the same way are for, as, since, though, due to, provided that, because of, unless, and so/so that.

Is since a causal conjunction?

Since is used as a causal conjunction (and has been since the 16th century) in the same way that because is used: Since you ate the ice cream last night, we don't have any dessert tonight.