I'm not sure "immediately" has a precise opposite. Maybe "eventually"? The opposite of "accidentally" could be "intentionally" or "deliberately". The opposite of "suddenly" could be "gradually", but it depends on context.

Beutiful antoynims word is my very hard please tell me

accidentally the opposite is wehne u programe to do sm thing we use i m going to do ;or planified to do sm thing immediately the opposite is lately and is the opposite for suddenly