간단하다 vs 쉽다
안녕하세요.
What is the difference between these two verbs ? Are they interchangeable?
감사합니다

Question

간단하다 vs 쉽다
안녕하세요. 
What is the  difference  between  these two verbs ? Are they interchangeable? 
감사합니다

Answer

Nope, they are not interchangeable at all :)
안녕하세요 is a polite form of saying hello, you may use it when you meet your elders, or formal occasions.
감사합니다 is also a polite form of saying thank you. The usage is similar but those two are totally different.

Answer

간단하다 is simple
쉽다 is easy

I think they can be interchangeable.