Final answer:
The correct sentence is: When I think about the rain, I know I must find an umbrella.
Step-by-step explanation:
The correct sentence is: When I think about the rain, I know I must find an umbrella.
In this sentence, the comma is correctly used to separate the dependent clause 'When I think about the rain' from the independent clause 'I know I must find an umbrella.'
The incorrect sentence 'When I think about the rain I know I must, find an umbrella.' contains a misplaced comma after the word 'must.'
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