This is a tough one! There are some better explanations in the internet if you Google “what is difference between say and tell?” My thinking is that “say” is more like “decir” and “tell” like “contar”. Would be interested in hearing what others say.

Gracias por sus respuestas.

Darren's explanation is spot on. Tell has a couple of other meanings also. You catch your big brother doing something that he shouldn't. You say I'm going to tell mom. Tell can mean to unmask or reveal something. Tell can also mean to detect or determine something. Your wife might say, "I have lost five kilos. Can you tell? That means can you detect the difference or observe the difference? Just in case you were wondering the answer to that question is always yes! That is called a sense of self-preservation. Other examples: One is genuine the other is an imitation. Can you tell the difference? Time will tell. This common expression means that time will reveal the truth.

One American friend told me that you can use 'say' when you are going to repeat the exact words someone said.