from now on,从现在起,一般用：“一般将来时；
from then on,从那时起,一般用：”一般过去时.
I will study hard from now on.
I remebered her words from then on.
from...on 是英语习惯用语,意思是“从所述的时间开始而延续的时间未定”.一般都是做时间状语.
1.from now on 是从现在开始,可以用一般现在时,一般将来时,
例：From now on you can work on your own.
From now on I will work harder and harder.
2.from then on是从那时开始,一般指过去的某时间.可用一般过去时,现在完成时（从那个时间点到现在）,
例:She never spoke to him from then on .
From then on I have got three toys from my father .