What type of clause is the given sentence?

Is the sentence "Then the staff assembled the units." a dependent or an independent clause?

Well, according to me, since there is subordinate conjunction "Then" in the sentence with no other sentence, so, therefore, it should be a dependent clause. And also it kinda sounds incomplete when we speak out loud. :)

But my book says that its the main clause or independent clause.

Any help would be appreciated.

Solution 1:

In this case then is operating as an adverb (Merriam-Webster), denoting that the action in the sentence has occurred at that time or after another action in a sequence:

: at that time

: following next after in order of position, narration, or enumeration : being next in a series

In this case its function as an adverb rather than a conjunction can be tested by changing the position of then to the end of the sentence or other positions. In these cases, the expression is still syntactically valid:

The staff then assembled the units.

The staff assembled the units then.

A subordinate conjunction has to stay in position at the start of its clause. Here's an illustration with because:

The event happened because the staff assembled the units

but not

:( The event happened the staff assembled the units because.