When using "That's" or "It'" in this sentence is more about the context of the conversation. Scenario: Using "That's" The sentence is open ended, (That's or That is) You: I see that you play the piano Me: Yes, I do. I have been playing the piano since I was 5. You: That's so cool! Scenario 2: Using "It's", describing the piano (It's or It is) You: I see you play the piano, It's so cool. This "It's or It is" is now describing that the piano; which is an object is cool So you think the piano is cool when you use it's.

Hi👋 I’m new here and thank you for being here to perfect my English.

It depends on context. Perhaps one way to think about it is this: (my opinion) 1. "It" generally refers to a single noun. 2. "That" can refer to a phrase, clause, situation, concept, or end result. Here are some examples: 1. I see you play the piano. That's cool! (that = the fact that you play the piano) 2. You cheated me. That's not fair. (that = the fact that you cheated me) 3. Everyone died in the end. That was not the ending I was expecting. (that = the result where everyone died) 4. I lost my wallet. Have you seen it? (it = wallet) 5. That's a nice piano. Where did you get it? (it = that piano) 6. There's a plane up in the sky. Can you see it? (it = the plane)