了 basics
The little particle that marks a finished action or a changed situation.
了 does not simply mean "past tense". It signals completion or change.
After a verb: the action is done
。
I ate (the meal).
。
He bought a book.
At the end of a sentence: a new situation
。
It has gotten dark.
。
I'm not going any more.
Not for everything
Habits and general facts usually do not take 了.
。
I drink tea every day.