Adverb clauses of cause or reason are introduced by the subordinating conjunctions because, as, since and that.
- I sing because I like singing.
- He thinks he can get anything because he is rich.
- Since he has apologized we will take no further action against him.
- As he was not there I left a message with his mother.
- I am glad that you have come.
- My parents were disappointed that I didn’t get the scholarship.
- He was furious that his book was panned by most reviewers.
The conjunction that is often omitted.
- I am glad you like it. OR I am glad that you like it.
- They were disappointed you weren’t in. OR They were disappointed that you weren’t in.
As and since are used when the reason is already known to the listener.
- As it is raining again we will have to cancel the match.
As and since-clauses are relatively formal. In an informal style, the same idea can be expressed with so.
- It is raining again, so we will have to cancel the match.
Because-clauses are used to give information which isn’t already known to the reader or listener.
- Because he had not paid the bill, his electricity was cut off.
Note that a because-clause can stand alone. As and since-clauses cannot be used like this.
- ‘Why are you looking at her like that?’ ‘Because she smiled at me.’ (NOT As she smiled at me.) (NOT Since she smiled at me.)