Dear
pramitmishra0607,
I'm happy to respond.
First of all, let me commend you, my friend, for asking an excellent question! It's clear that you put tremendous thought and effort into the question you are asking, and that's wonderful. Asking excellent questions is one of the habits of excellence!
The answer to this is subtle. The word "
although" is a subordinate conjunction, and as such, it introduces a full clause, a dependent clause. It must be followed by a full clause. The catch, though, is that small words, e.g. [pronoun] + ["to be" verb], in the clause can be dropped if they are implied, so what is printed on the page will not
look like a full clause, but it really is a full clause.
For example, from SC13 above:
Though it is called a sea, the landlocked Caspian is actually . . .
The words "
it is" are implied, and with those, what follows "
though" is a full clause.
From SC36 above:
. . .
it is indicative that the economy, although it is growing slowly, is not nearing a recession.
Again, the word "
it is" are implied, making what follows "
although" a full clause.
In SC17 above, there's no way to put in a pronoun plus an auxiliary verb to turn what's following the "
although" in (C) & (D) into full clauses.
(C)
appear on the surface of the Sun as dark spots although they never are sighted at the Sun's poles or equator.
That doesn't work. The word "
sighted," by its very nature, is a participle that is not part of an ordinary full verb. (D) is even worse: I have no clue what anyone would put in front of "
having been sighted"---we can't really change perfect participles into full verbs.
That's the difference between, on the one hand, SC13 & SC36, and on the other hand, SC17. In the former two, we can insert an implied [pronoun] + [auxiliary verb] to turn what's there into a full verb. In the latter, we do not have that option, so the construction is wrong.
Does all this make sense?
Mike