[Grammar] Is "this is" a main clause?

[Nanu1]

"This is because they have already gone home". In this sentence "because they have already gone home" is a dependent clause. Genarally, a subordinate clause normally follows the main clause. Here is my question. Is "this is" a main clause?

 

[Matthew Wai]

"This is because they have already gone home".

'This' is the subject. 'Is' is the main verb. 'Because they ...', a noun clause, is the subject complement.
That's how I would parse it.

Is "this is" a main clause?

I don't think so.