since then 和from then on用法区别

Question

都表示从那时起，但用法(语法，位置)上有何差别

Answer 1

since then 只能用现在完成时，“自从那时到现在为止”
from then on 可以用于现在完成时，过去式或者将来时，要看具体的语境
例如：He borrowed a book from the library three weeks ago. He has kept it from then on.

The final exams will ends on June 30. We'll begin our summer holiday from then on.
Tom died in 1899, from then on his son had a bad life.