word choice - Usage of 'from' and 'since'

I am often confused with the usage of since and from.

I know him from 15 years

I know him since 15 years.

Which one is correct? How would I decide which is the right preposition to use?

Answer

'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 9 in the morning to 6 in the evening

Quite close is 'since' and 'for'

'Since' and 'For'

Okay, as I said, 'since' talks about a particular point. So, you need to mention "since" which date the thing has 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
I have been living in this house since 2005

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