Questions about example sentences with, and the definition and usage of "Sadly"

A: Unfortunately

A: 슬프게

A: Sadly, her cat passed away
Unfortunately her cat passed away.

There’s no big difference except for the fact that sadly can only be used at the start of a sentence.

A:
They are essentially the same but there is a subtle difference.
Sadly means that something bad has happened to someone, or you, that makes a change in plans.
Unfortunately means that something happened that changes plans, not necessarily bad for someone.

For example:
I cannot attend your party because sadly my aunt has died and her funeral is on that day.
I cannot attend your party because unfortunately my father needs my help at work on that day.

A: sadly e unfortunately, tem mesmo significado.

exemples:

Sadly, she passed away last night. ( Infelizmente, ela faleceu na noite passada).

Unfortunately, você usa com uma negação na frase.

Unfortunately, it is going to rain tomorrow, and I will not go there. ( Infelizmente, vai chover amanhã e não irei ir lá).

Unfortunately, my passport has expired, and I will not travel to Ireland.

A: Similar in meaning but I believe that these are taken literally 'unfortunately' implies bad luck while 'sadly' implies...sadness/regret.