For and Since for Time
We often use for and since when talking about time.
for + period
A period is a duration of time, for example: 5 minutes, 2 weeks, 6 years. For means "from the beginning of the period until the end of the period." For can be used with all tenses.
since + point
A point is a precise moment in time, for example: 9 o'clock, 1st January, Monday. Since means "from a point in the past until now." Since is normally used with perfect tenses.
for
a period
(from start to end)
>===< since
a point
(up to now)
x===>|
for 20 minutes
for three days
for 6 months
for 4 years
for 2 centuries
for a long time
for ever
etc since 9am
since Monday
since January
since 1997
since 1500
since I left school
since the beginning of time
etc
all tenses perfect tenses only
For can be used with all tenses. Here are a few examples:
They study for two hours every day.
They are studying for three hours today.
He has lived in Bangkok for a long time.
He has been living in Paris for three months.
I worked at that bank for five years.
Will the universe continue for ever?
For is NOT used with "all day", "all the time" etc.
I was there all day. (not *for all day)
Since is normally used with perfect tenses:
He has been here since 9am.
He has been working since he arrived.
I had lived in New York since my childhood.
Since can also be used in the structure "It is [period] since...":
It is a year since I saw her.
How long is it since you got married?