When do we use "for" and when "since"?

Air pollution has been bad for the last week.

Is it possible to say since last week? If not what would you say using the word since?

For is used with a period of time as in for the last week/year/month.

Since is used with a point of time as in since last week/year/month.

for three hours
in contrast to
since three o’clock


As explained by others; ‘for and since’ are used to refer to a period or point of time.
for: a period of time e.g.: for a month; for three hours etc.
since : expressing time factor with a point of time;
e.g. since six o’clock; since Wednesday last etc.
Apart from this, it should be noted that they are especially used in perfect tense forms.

I was asking about last week, looks like it can be a point of time and also a period. I like Alan’s explanation. Now I can see the difference between ‘for the last week’ and ‘since last week’. That’s all I wanted to know. The article ‘the’ makes the difference.
But thanks anyway.

I think still you have not clearly understood the use of these two words-for and since.

For: a period of time. For a week; for a month; for three hours etc.

Since : since last year; since last week; since three o’clock

I have been wating for you for the past three hours.
I have waited here since 9 in the morning expecting you to meet me here and take me home.
I have been working in this company since 1997.(You are still with them.)
I have worked for this company for more than eight years.(This also means that you are still with them.)

Dear Narayanan,
Thank you very much for taking your time and explaining things to me and others. That’s very nice of you.