y then / by that time中的by是“到……为止”,常与一般过去时、过去完成时、一般将来时、将来完成时连用.We will have finished the task by that time.
since that time/ since then中的since是“自从……以来（直到现在）”,常与现在完成时连用.She has lived in the city from then on.
from then on从那时起,常用于一般过去时.I remebered her words from then on.
from now on从现在起,常用于一般将来时.I will study hard from now on.