Actions or states in the positive or negative sentences are relative to time period they are at. This topic is useful when we study differences between tenses, sequence of tenses and indirect speech etc.
Finished (period of) time
last year, last month,
one millisecond ago,
a hundred years ago,
on the 11th of August,
from 1998 to 2003,
at 5 in the morning, from 2 to 5, between 4 and 7 pm, etc
We went to the movies at 4
Unfinished (period of) time
this year, this month,
this morning (if it’s before noon at the time of speaking)
They have been to Paris this year.
Unspecified (period of) time
Before the present time:
have your own? contact me
She has not seen this movie before
Unfinished and finished time compared
today – yesterday
this year – last year
now – then
Don’t mix this topic with always, never, often, etc. These characterize the frequency of actions and states.
More on Introduction to Tenses: