by 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.
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