否定性动词cannot也可以被写为can not，但前者一般更适当。Can not 形式特别着重表达的是这个动词的否定性，因此仅在想要表达这种强烈意思的时候用can not才适当。而这一般有两种情况，一是来表达特别重要的主张，一是当not表达的否定意义与读者的期望相反时。通常如果不是此类情况，则应该用cannot。
以下我们举出两个例子来说明恰当使用can not的典型情况。
- Francis et al. claimed that these tubules dilate sufficiently that they can allow a flow rate of 3 cm3/s, but our experiments clearly demonstrate that they can not. In fact, we have found that even under extreme conditions, the maximum flow rate is less than 1 cm3/s.
- The main finding of this study is that for a > ac, when computing the velocity, we can not ignore the nonlinear coupling of modes.
