Welcome, Dolomite.
All are OK, I agree. They differ though. In [1b]
who is the subject of
will be, whereas in [2b]
who is a
subject complement:
[1a]
Who / do you think / will be the next president?
[1b]I think
who will be the next president
[2a]
Who / do you think / the next president will be?
[2b] I think
the next president will be who
The same holds true for the other two examples.
Who has been moved out of the subject position in [3] and out of the subject complement position in [4]:
[3]
Who / do you think /
(WHO) will be the next President of the United States?
[4]
Who / do you think / the next President of the United States will be
(WHO)?
Note that, clauses are boundaries and the more words that a WH-word moves across results in a more complex parsing pattern for the reader / speaker. So, according to some people, the closer the WH-words is to its original position, the better.
#1 Statement
You think that
Sam will be ...
Question
You think that
who will be ... ?
Who do you think will be .... ?
Here the WH-word
who replaced the noun
Sam and moved out of a
that-clause to the front of the construct, and it's only a few words away from its original position. Notice the word
that isn't possible in the question example:
Ex: Who do you think
that will be ...
The reason it doesn't work,
that functions as a conjunction, not as a subject, so its presence above doesn't fit the grammar. A conjunction joins two like forms, but in our example,
who do you think and
will be are different, so
that can't conjoin them.
In our second example, however, the conjunction
that is possible:
#2 Statement
You think that the next president will be
Sam.
Question
You think that the next president will be
who?
Who do you think (
that) the next president will be?
Notice also that
who moved out of the
that-clause, just like in our first example (#1), but the amount of words it crossed is significantly greater. Moreover, in our first example
who only had to passed one verb,
think, whereas in our second example (#2),
who passed over two verbs,
will be and
think, leaving its verb stranded at the end of the sentence. That's a problem, for some.
In short, some people feel that leaving a copular verb stranded (i.e., without its subject complement) at the end of a sentence is not necessarily ungrammatical, but more so that it's in poor style because it violates the structure's integrity (#2), whereas others think it's perfectly acceptable to leave the verb stranded, especially if its subject complement can be traced, found in the sentence (#2).
So you see, both #1 and #2 are acceptable.
You be the judge.
