For – जब समय की अवधि का उल्लेख हो या जब digit आता है तब ‘For’ का प्रयोग करें |
Example :
1. For two hours, for last two year
2. For five days, for last 2 months
3. For 10 years, for last 3 week

Since – जब शुरुआती समय का उल्लेख हो, तब since का प्रयोग करें |
Example :
1. Since Monday, since the beginning
2. Since 2008, since time immemorial
3. Since 7 P. M., since last year