A red flag is a warning sign that something is wrong with a person or situation, especially in a relationship. Literally, a red flag is a piece of red cloth used as a warning signal. The expression is neutral enough for conversation and IELTS Speaking, but in an essay, warning sign is more precise.
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be a red flag (for + person); raise a red flag / red flags; red flag behaviour
- He cancelled three dates in a row, which is a red flag for me.
- Ignoring your messages for days isn't being busy, it's a red flag.
- Her friends warned her about the biggest red flag: he never apologises.
What changed
First versionHis controlling behaviour made a red flag for her.
RevisionHis controlling behaviour raised a red flag for her.
In English, a warning sign is raised, like an actual flag: raise a red flag, the same way you raise concerns. Make might look like a good verb for causing a feeling, but it simply doesn't combine with red flag. A simpler option also works: His controlling behaviour was a red flag for her.
A red flag is an early signal, not the problem itself: a small lie or jealousy without reason can be worrying long before any serious conflict. The expression has a counterpart, green flag, a trait that speaks in someone's favour: He always keeps his promises, that's a green flag.
Your turn
Checking your partner's phone without asking should ___ a red flag straight away.
Show the answer
raise — The fixed expression is raise a red flag, meaning to set off an alarm: checking a partner's phone without asking should raise one right away. Make a red flag sounds like literally crafting a flag, and that's not how a warning sign is described.
Describe a trait in someone's behaviour (not necessarily in a relationship — work or friendship works too) that worries you, and explain why.
Whenever my colleague avoids answering direct questions, it's a red flag for me because I can't trust what he says next.
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