Final answer:
The sentences are rewritten using the phrase 'You'd better' to express advice or warnings. This format expresses a strong suggestion for necessary action.
Step-by-step explanation:
To rewrite the given sentences using 'You'd better', we can phrase them as follows:
- You'd better turn off your cellphone as the plane's about to take off.
- It's very cold outside. You'd better wear a jacket.
- You have been feeling ill for the past three days. You'd better see a doctor.
- You've had too much to drink already. You'd better not order another one.
- There's a red flag up. You'd better not swim in the ocean.
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