Question

while declining the invitation polite language should be used.​

Answer 1

Explanation:

Apologise for declining

You can say something like “Sorry I can't attend” or “I'm afraid I'm not able to come.” These expressions work equally well in spoken or written English so you could use them on the phone or in an email.

Answer 2

Answer:

because it is to be say polite way otherwise he would not invite you again