1)Since i was unwell ,so i did not come.
2)Since i was unwell ,i did not come.
My book said sentence 1 is not correct as we don't use so with since. Is it correct?
1)Since i was unwell ,so i did not come.
2)Since i was unwell ,i did not come.
My book said sentence 1 is not correct as we don't use so with since. Is it correct?
Sentence (1) is incorrect. I don't think the explanation "we don't use since with so" explains that much, though.
The sentence is incorrect because it doesn't have a main clause. The word so is a coordinating conjunction that connects two main clauses, and the word since is a subordinating conjunction that introduces a dependent clause. You can't make a complete sentence by using a coordinating conjunction to connect a dependent clause to a main clause.
*Because I was unwell, and I did not come.
*Though I was unwell, but I came anyway.
are incorrect sentences for exactly the same reason. You need to get rid of one of the conjunctions.
Yes, "we don't use so with since" simply means the sentence can do either without since or without so.
In other words,
I was unwell, so I did not come.
(without since) would also be correct.