People use "then" to finish a sentence like this:
If you put the couch here, then it will be blocking one of the the outlets.
"Then" shows the result of a possible action. Sometimes people leave out the first part of the sentence, so it turns into this:
Then it will be blocking one of the outlets.
You can complete the sentence with a verb like this:
A: Let's ask the professor for an extension.
B: No! Then she'll know that we've just been slacking off.
Or you can complete the sentence with "be + adjective":
I thought about going for a run, but then I'll be all hot and sweaty.