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.