As and Since Grammar Usage
Hello
According to what my English teacher has taught, as and since are conjunctions of reason and we only use a comma if they're used at the beginning of a sentence.for example: Since you are unable to answer, perhaps we should ask Jane./ As he was too tired, he didn't feel like going to the gym.
Then,
Why is there a comma in this sentence?!
We asked Philip to come with us,as he knew the road.
What is the grammatical note of the sentence above?
Please help a.s.a.p
Thanks