It's a pretty elementary grammatical rule. Just trying to help you understand it, and possibly prevent you from looking silly in the future!
I think the only one looking silly is you making grammar corrections on a coin post. Not only on my post but others as well.
Regardless of your intent, the guy clearly doesn't like it, so just let it go. If you truly feel the need to help someone, there are many more on this board who could genuinely use it.
Seriously, it is a coin post not a grammar lesson. Unless you have something to add coin wise you are being petty
You are the only one that looks silly. Take the childish post somewhere else. This forum is not the place for it. This is a coin forum.
A PM to the OP would have sufficed if you really wanted to help him. Then he could have taken your lesson as a helpful suggestion, or said "thanks, but no-thanks", but it would have been private instead of derailing this thread with a silly grammar lesson that sounds like you really just wanted to be condescending.
I have a 1983 DDR in lower MS grade, but yours is one to beat all - I didn't know there were such gorgeous specimens in existence.