Can we use "there is" a main clause? and independent clause starts with a word "then"?

Question

1."This is because they have already gone thier home". In this sentence "because they have already gone thier home" is an dependent clause of independent clause "there is". Here my doubt is "there is" main clause?
2."We went to the movie and then we went to restaurant near the theatre". In this sentence "then we went to restaurant near the theatre" is a independent clause. Here my doubt is, can we start independent clause with "then"?

Answer 1

This is for a good reason. 
This is because they have already gone home. 

The first line represents a sentence with a single independent clause.  We can divide the clause into a subject and a predicate.  We can divide the predicate into a verb and a complement. 

The second line represents a complex sentence, with both an independent clause and a dependent clause.  The independent clause has the same three parts as the clause in the first line: the subject "this", the verb "is", and the complement "because they have already gone home".  This exactly matches the earlier subject "this", verb "is" and complement "for a good reason". 

"For a good reason" isn't a clause.  It's just a prepositional phrase.  It should be easy to see that it is a part of the independent clause on the first line.  The same thing is true about the entire dependent clause on the second line -- the dependent clause is a part of the independent clause.  One clause exists completely inside another clause: 

{ This is ( for a good reason ) }. 
{ This is ( because { they have already gone home } ) }. 

Do the words "this is" on the second line just above represent a complete independent clause?  No, or at least they don't represent the same clause.  All of the words on the second line are parts of the independent clause.  "This" and "is" are the two words that belong only to the independent clause.  The rest of the words belong to both the independent and the dependent clauses. 

A dependent clause isn't simply attached outside an independent clause.*  It plays a role inside.  If you take a dependent clause away from its matrix clause,** then what you're left with is either an incomplete clause or simply a different clause with a different meaning. 

 

We've had a long evening.  First we went to a movie.  Then we went to a restaurant near the theater.  After that we came here.  Now we're ready to go home. 

It's no stranger to start an independent clause -- or even a complete sentence -- with "then" than it is to start one with "first" or "now".  Like "first", "then" can indicate a position in sequence.  Like "now", "then" can indicate a position in time. 

However, don't confuse "then" with "than".  The word "than" will not appear at the start of an independent clause.  Just like "because", it is a subordinating conjunction.***  It has the power to turn an independent clause into a dependent clause. 

The two words may look similar, but they behave very differently. 

_______________ 

* For what it's worth, some dependent clauses are simply attached on the outside.  One example is the "for what it's worth" in the prior sentence.  However, it's simpler to call these clauses supplemental rather than dependent,  since they add meaning without depending on having a role inside some other clause. 

** "Matrix clause" simply means a clause that contains another clause.  There is no maximum depth: an independent clause can contain a dependent clause that itself contains yet another dependent clause.  In that case, the one dependent clause is the matrix of the other dependent, even though the matrix is not independent.

*** It might make more sense to call it a subordinating preposition, but I'm trying to stick to the terms that traditional grammar has handed me.  "Than" also functions as an ordinary preposition, as in "I like this better than that."