noun
BAD : I was fool to believe him.
GOOD : I was a fool to believe him.
GOOD : I was foolish to believe him.
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Fool is a countable noun and refers to a person: 'She thinks I'm a fool for lending him the money.'
The adjective is foolish : 'You were right. It was foolish of me to lend him the money.' 'This foolish idea could ruin everything.'