I’ve always tried to use these two words appropriately, and so I looked up their definitions and common usage only to find that there’s no real clear difference, and they are often used interchangeably. But if there are two distinct words, surely there should be two distinct usages?  So, from my experience, and my research, here’s how I make the distinction.

When you express sympathy with someone, you understand their experience without necessarily having had the same experience yourself. You can imagine what it’s like to be in their shoes, but haven’t, so you are sympathetic.

To be empathetic with someone, you have actually had the experience, or something so closely similar, that you know how they feel, and so your sharing is one of intimate and personal knowledge.  You don’t have to imagine what they feel like, you know from personal experience.