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The referee blew his whistle to signal that there was a penalty.
"when the whistle blew"
It's the predicate. The subject would be whoever did it. e.g. Kelly blew her whistle Kelly=subject blew her whistle=predicate
The verb "blew" in the sentence "the watchman blew this whistle" is transitive because it requires an object ("this whistle") to complete its meaning.
He blew a whistle to signal the start of the race.
The wind blew my coat out of my hands.
It means she told on them.
He liked to whistle as he walked.The whistle signalled the end of the match.
Wirey Willy wrapped Walter up when the whistle blew.
No, she can't whistle 'cause she hasn't even puckered her mouth up and she never blew very gently.
The main idea of the sentence is 'I stopped the car', so this is the independent clause. 'When the whistle blew' gives additional information, but isn't meaningful alone. It is the dependent clause.