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

[Ella]

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?

[Alan]

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.
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[Michael]

for three hours
in contrast to
since three o'clock

Mike

[Narayanan Krishnaswamy]

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.

[Ella]

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.

[Narayanan Krishnaswamy]

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.)

[Ella]

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