A complex sentence has two parts: a subordinate clause and an independent clause.
The subordinate clause always begins with a subordinating conjunction.
Today we’ll talk about two subordinating conjunctions that can begin a complex sentence: as and while.
As I was walking to school
This is a clause beginning with the subordinating conjunction as. It is not a complete sentence because we don’t know what happened as you were walking to school. We must add an independent clause to make the sentence complete.
As I was walking to school, my shoelace came untied.
As I was walking to school, I heard the birds singing.
Notice that a comma goes between the subordinate clause and the independent clause.
Here are more sentences beginning with the subordinating conjunction as:
As the bus pulled up, three kids came running toward it.
As the bell was ringing, the teacher stepped into the room.
As the sun went down, the air got colder.
The word while is another subordinating conjunction. Here are sentences that begin with while:
While it was raining, I cleaned the house.
While we are together, we should make plans for next summer.
While the noodles boiled, Mom set the table.
Anytime you begin a sentence with the subordinating conjunctions as or while, you must put a comma after the subordinate clause and then add an independent clause.