In the sentence"
It would be much nicer if I could go with you.
S V SC real S.
In terms of grammar, the real subject of the sentence is "if...you", right?
so, "if... you" is a noun clause instead of a adverb clause, right?
Adverb clauses are adverb clauses whether they come at the beginning of a sentence or at the end.
If I could go with you, it would be nicer.
It would be nicer if i could go with you.
Let's try to identify "it." That's the subject.
The trip would be nicer if I could go with you.
If I could go with you, the trip would be nicer.
Many of us use "it" too much. I'll often go back into my writing and substitute what I actually mean for the "it." IT makes the writing more clear.