Short answer: Use cannot in almost every sentence. It is the standard spelling in both American and British English. Can not is not a mistake, but it fits only in a few cases, covered below.
What “cannot” means
Cannot is the negative form of can. It means “is not able to” or “is not allowed to.”
- I cannot open this file.
- You cannot park here after 6 p.m.
- She cannot attend the meeting on Friday.
Dictionaries list cannot as the normal form. It is the safe choice in emails, essays, reports, and school work.
When “can not” is correct
Write can not as two words when “not” belongs to the words that come after it, not to “can.” The most common case is “not only… but also.”
- You can not only save time but also cut costs.
Here “not only” is one unit. If you wrote cannot only, the sentence would read oddly. The same thing happens when you want a clear pause before “not.”
- Will you help me? I can. Or I can not, if you prefer to do it yourself.
Some writers also use can not to put extra weight on the negative: “You can not be serious.” Most editors would still prefer cannot here, so treat this use as a style choice.
A quick test: if “not” negates “can” itself, write cannot. If “not” starts a phrase like “not only” or “not just,” write can not.
Cannot vs can’t
Can’t is the contraction of cannot. It means the same thing but sounds more casual.
| Form | Best for |
| cannot | Formal writing, instructions, rules, official messages |
| can’t | Everyday speech, chats, informal emails |
| can not | Rare cases such as “can not only… but also” |
- Formal: Applicants cannot submit the form after the deadline.
- Casual: I can’t find my keys.
Don’t confuse can’t with cant. Without the apostrophe, cant is a different word, meaning insincere talk or a slant.
How the two sound
Spoken, cannot usually puts the stress on the first part: CAN-not. In can not, the word “not” is stressed: can NOT. That stress difference is why can not suits emphasis.
Common mistakes
Writing “can not” out of habit. Many people split the word because can and not look like two separate words. In most sentences this looks like a spelling slip.
- Not ideal: We can not accept late entries.
- Better: We cannot accept late entries.
Writing “cannot” before “not only.” This makes the sentence hard to read.
- Not ideal: You cannot only read the guide but also test the steps.
- Better: You can not only read the guide but also test the steps.
Using “cannot” with “but.” The phrase “cannot help but” is common and accepted: I cannot help but laugh. Some people find it wordy and prefer “I can’t help laughing.”
Quick rule to remember
Write cannot by default. Switch to can not only when “not” starts a phrase like “not only” or when you want a clear pause before the negative.
FAQs
Is “can not” grammatically wrong?
No. It is a valid spelling in the right context. It is just much less common than cannot.
Which is more formal, cannot or can’t?
Cannot is more formal. Use can’t when you want a relaxed tone.
Do British and American English differ on this?
Both use cannot as the standard form, so the choice does not depend on region.
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