We haven’t spoken to each other since yesterday.

Since when have you been an expert on that?

I haven’t danced the twist since the ’60s.

As a subordinating conjunction, since will begin a dependent clause that must be joined to an independent clause.

Since the new teacher came to our school, students have been trying madly to get into her class.

I will be over after 6:00 since I don’t get off work until 5:00.

Other words which can be both prepositions and conjunctions will fit into sentences in the same way.

Remember that if you begin a sentence with a subordinating conjunction you MUST place a comma following the dependent (also called a subordinate) clause.