adverb
used to indicate that something is regrettable or unfortunate
preposition
unfortunately can be used as a preposition in some cases to indicate a negative or unfortunate circumstance
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Unfortunately, the project was delayed due to unforeseen circumstances.
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I'm sorry to say that the concert has been canceled, unfortunately.
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Bummer, we can't go to the beach today, unfortunately.
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The dark clouds gathered overhead, unfortunately signaling the impending storm.