thankful vs. grateful

[thedaffodils]

#1: I'll be thankful if you help me. (Wrong)
#2: I'll be grateful if you help me. (Correct)

Hi! Could you please tell me why " thankful" is wrong in #1? What is the difference between " thankful" and "grateful"?

Thanks in advance!

[banderas]

Hi! Could you please tell me why " thankful" is wrong in #1? What is the difference between " thankful" and "grateful"?

Thanks in advance!

The question is quite tricky...
I will be grateful if you help me= I will appreciate your help and say thank you.

I will be thankful if you help me= I will be relieved if you help me.

You can be thankful/relieved if something unpleasant has not happened or has ended like in:
"I was thankful that the meeting didn't last long, because I had a train to catch."
That's way "I will be thankful" does not work in your example.

[thedaffodils]

Hello Banderas, you are helpful. Thank you very much! I understood.