unbelievable: so surprising that it is almost unable to believe
[word: definition]
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Q1: What does 'it' refet to?
Q2: Is 'to believe' a real subject?
Is 'it' an empty(artificial) subject?
The definition needs a slight correction. It should read like this:
unbelievable: so surprising that it is almost unable to be believed
[word: definition]
The pronoun "it" seems to be referring to whatever noun the adjective "unbelievable" would describe in a sentence. Here's an example:
His arrival at home yesterday was unbelievable. We all thought he was still in Germany.
The noun "arrival" is the noun being described by "unbelievable."