I generally don't follow the terminology and framework of A Comprehensive Grammar of the English Language (CGEL) by Quirk et al., rather I follow The Cambridge Grammar of the English Language (CaGEL). But at times I do read CGEL. It is during one of those reading session I came across this:
only the adjunct clause can be the focus of a cleft sentence:
1. It's because they are always helpful that he likes them. [CORRECT]
2. It's since they are always helpful that he likes them. [INCORRECT]
I found this on page no. 1071 where they were distinguishing between Adjunct and Disjunct. Now CaGEL doesn't have any disjunct category and after reading CGEL it sounds like their disjunct is basically CaGEL's speech-act related adjunct. But still I can't make out how "since they are always helpful" is a disjunct and "because they are always helpful" is an adjunct.
There is another example of disjunct on CGEL's page no. 612:
Since she ran out of money, she had to defer buying a new car.
Since she ran out of money is a disjunct.
I knew in CGEL there is a disjunct category, but it so far didn't make much sense to me. I was not even bothered until I find the reason of incorrectness of sentence #2.
Can you please tell me what disjunct is claering out my confusions I already mentioned?
Why in sentence #1, because they are always helpful is an adjunct and in sentence #2, since they are always helpful is a disjunct?
Without the disjunct category, following CaGEL framework, can you please explain why sentence #1 is correct and why sentence #2 is incorrect?