## 'Since' and 'From'##

'Since' talks about one particular point of time till now. 

> I have been working ***since*** 7 am

'From' in *most of the cases* talks about the starting point and ending point. Said that, the format could be 'from....to/till/until'

> The shop is open ***from*** morning 9 ***to*** evening 6

Quite close is 'since' and 'for'

## 'Since' and 'For'##

Okay, as I said, 'since' talks about a particular point. Said that, you need to mention which year/period the things have been happening.

On the other hand, 'for' is used for a total period of time and you don't need to define a starting point. 

The best example I can think of is...

> I have been waiting ***for*** two hours

but...

> I have been waiting ***since*** 2 o'clock

In the latter example, you need to define the starting point. 

Another example:

> I have been living in this house ***for*** 10 years <br/> I have been living in this house ***since*** 2005

So, in your case, you have known him ***for*** past 15 years OR ***since*** 2000