Cambridge dictionary online wrote:
then (NEXT) Show phonetics
adverb
next or after that:
Let me finish this job, then we'll go.
Give her the letter to read, then she'll understand.
This is from the Cambridge dictionary online.
[Independent clause], [then] [independent clause.]
Having then (in the context we're using it here)before an independent clause makes it a dependent clause.
From the OWL at Purdue University:
When you want to use commas and semicolons in sentences and when you are concerned about whether a sentence is or is not a fragment, a good way to start is to be able to recognize dependent and independent clauses. The definitions offered here will help you with this.
Dependent Clause
A dependent clause is a group of words that contains a subject and verb but does not express a complete thought. A dependent clause cannot be a sentence. Often a dependent clause is marked by a dependent marker word.
When Jim studied in the Sweet Shop for his chemistry quiz . . . (What happened when he studied? The thought is incomplete.)
Dependent Marker Word
A dependent marker word is a word added to the beginning of an independent clause that makes it into a dependent clause.
When Jim studied in the Sweet Shop for his chemistry quiz, it was very noisy.
Some common dependent markers are: after, although, as, as if, because, before, even if, even though, if, in order to, since, though, unless, until, whatever, when, whenever, whether, and while.
From another part of the OWL at Purdue University:
Pattern Five: Complex Sentence
This pattern is an example of a complex sentence with a dependent marker.
Dependent marker dependent clause[ , ] Independent clause[ . ]
Examples of dependent markers are as follows: because, before, since, while, although, if, until, when, then, after, as, as if.
Example: Because doctors are concerned about the rising death rate from asthma, they have called for more research into its causes.
Pattern Six: Complex Sentence
This pattern is an example of a complex sentence with a dependent marker.
Independent clause dependent marker dependent clause [ . ]
Examples of dependent markers are as follows: because, before, since, while, although, if, until, when, then, after, as, as if.
Example: Doctors are concerned about the rising death rate from asthma because it is a common, treatable illness.
http://owl.english.purdue.edu/owl/resource/604/01/
http://owl.english.purdue.edu/owl/resource/598/01/
The only time then can be used at the beginning of an independent clause is if it being used as "at that time".
I was working in the city then. Then I was working in the city.
I was working in the city at that time. At that time I was working in the city.
This is one of those "I need to check with someone with the original copies of the novel" moments.
If "then" is being used as in "I walked to the supermarket, then I walked home", then the sentence has some problems. It doesn't really make sense because "I can" is just a statement of condition. It's like saying "My car is not broken, then I can drive". There isn't a first part of time for the then to come after.
If "then" is being used as in "at that time", it still doesn't make much sense; "I can" still doesn't have a time element to it.
For the change I made, it was under the assumption of an "if I do, at that time" sort of sentence.
I can imagine Nagato in a black pointed cap, but(, if I do,) then I see a scene where she tells a couple, "You two will break up in fifty-eight days three hours and five minutes."
Really, it is most clear as something along the lines of:
I can imagine Nagato in a black pointed cap, wearing a black cape and carrying a crystal ball, but if I do, I can't help seeing a scene where she tells a couple, "You two will break up in fifty-eight days three hours and five minutes."
This is a more clear version than the last one I suggested. It completely cuts out the ambiguity from the "then," and it also cuts the second imagine; it was a bit repetitive before.
Another option is what I suggested earlier:
I can imagine Nagato in a black pointed cap, but(, if I do,) then I see a scene where she tells a couple, "You two will break up in fifty-eight days three hours and five minutes."
The then is an "at that time" then. It could be technically correct with the semicolon, but the but makes it more clear to the meaning.
The last option is the "I did x, then I did y" type sentence.
I imagine Nagato in a black pointed cap, then I imagine a scene where she tells a couple, "You two will break up in fifty-eight days three hours and five minutes."
Very dry and barely makes sense.
You were complaining about my use of ,but. It isn't saying the two parts of the sentence contradict each other; rather, it is saying "If I imagine x, I have no choice but to imagine y."
It really works fine either of the two ways I suggested here; the third option is really not good at all, and it surely is not what was intended.
I'm going to leave it how I first changed it. If it is still too ambiguous, it can be changed to the "I can imagine Nagato in a black pointed cap, wearing a black cape and carrying a crystal ball, but if I do, I can't help seeing a scene where she tells a couple, "You two will break up in fifty-eight days three hours and five minutes."" option.