Sadly/Unfortunately he wasn't willing to cooperate.
Is it perfectly natural to use "sadly/unfortunately" at the start of the sentence like this?
Are they interchangeable in this context?
Which would be more natural in everyday speech?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
Although both adverbs convey the same idea, they don't work equally well in all contexts:
Unfortunately, he wasn't willing to cooperate and, as a result, the whole deal collapsed.
Here, unfortunately is used in the sense of the misfortune that results from his failure to cooperate.
Sadly, he wasn't willing to cooperate with the doctors trying to save him and died within a few weeks of the accident.
Here sadly is used because of the sad result of his failure to cooperate - his death.
You can't say whether they work equally well in your example without knowing the context.
Similarly, it's the context that would guide the speaker's choice.