Cool MateI don't know vs I'm not sure
When to use each in English, with meaning, register, and examples.
I don't know
Top 1,000 (very common)
I'm not sure
High-frequency chunk
| I don't know | I'm not sure | |
|---|---|---|
| Pronunciation | 🇬🇧 //aɪ dəʊnt nəʊ//🇺🇸 //aɪ doʊnt noʊ// | 🇬🇧 //aɪm nɒt ʃʊə//🇺🇸 //aɪm nɑt ʃʊr// |
| Meaning | I am not sure. | I don't know if it's true or not. |
| Example | What time is the meeting? I don't know. | I'm not sure if it will rain tomorrow. |
| Register | Neutral | Neutral |
| How common | Top 1,000 (very common) | High-frequency chunk |
| Collocations | I don't know what, I don't know why, I don't know how | I'm not sure about that, I'm not sure if, I'm not sure what to do |
| Antonyms | I know, I am sure, I understand | - |
| Common mistakes | Using it too formally in professional settings., Saying 'I not know' instead of 'I don't know'., Confusing it with 'I don't know how'. | Using it too formally in writing., Confusing it with 'I'm not certain' - which is slightly more formal., Overusing it in situations that require decisiveness. |
| Usage notes | Use in conversations when unsure about an answer. Avoid in formal situations where providing a response is expected. | Used when expressing doubt about something. Common in conversations but less formal in writing. |
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Frequently asked questions: I don't know vs I'm not sure
What's the difference between I don't know and I'm not sure?
I don't know: I am not sure. I'm not sure: I don't know if it's true or not.
Can you show an example of each?
I don't know: What time is the meeting? I don't know. I'm not sure: I'm not sure if it will rain tomorrow.
Can I use I don't know and I'm not sure interchangeably?
Not always. I don't know and I'm not sure are related and overlap in some contexts, but they differ in register, how common they are, and usage, so swapping one for another can change the meaning or tone. Check the differences above before substituting.