I feel sick because of you.
I caught the flu after being exposed to someone who was sick.
I knew that I would also disclose my location to the enemy if I were to call out to my team. I would like the first person with the answer to call out.
No, starting a sentence with "then" does not make it a complete sentence on its own. It is typically used as a transition word indicating sequence or consequence and should be followed by an independent clause to form a complete sentence.
The hunter stalked silently through the forest, eyes trained on its prey.
I feel sick because of you.
I was sick yesterday, but I have recovered.
The sick man asked the doctor to make a house call.
I was sick, but my brother was sicker, and our sister was the sickest of us all.
I am getting sick of answering your questions!
He has no compassion for the poor or the sick.
the nurse identified he was sick
I caught the flu after being exposed to someone who was sick.
Don't guzzle that water; you'll get sick!
i was absent yesterday because I was sick that day
I needed a representative to do my presentaion because I was sick.
the sick child was pitied