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plThe modal verbs néng 能, huì 会/會 and kěyǐ 可以 all occur before verbs they refer to. In some situations, they can all be translated as “can”. They differ in terms of focus: néng 能 emphasizes “possession of an ability” or “possibility related to the circumstantial conditions,” whereas huì 会/會 highlights “a learned or acquired ability,” and kěyǐ 可以 is used in contexts related to giving or asking for permission. In addition, huì 会/會 can also indicate a possibility that is perceived as the prediction of something to happen. In this sense, it can be translated as ‘will’ in English.
Differences
1. néng 能 and huì 会/會 expressing ability or capability
When talking about an ability or a capability, néng 能 or huì 会/會 can be used. In this case, néng 能 emphasizes “possession of a certain ability” or “possibility related to the circumstantial conditions,” as if saying “to be capable of…”, whereas huì 会/會 implies that the ability or skill is learned or acquired as if saying “to know how to…”.
Here, néng 能 cannot be replaced with huì 会/會, because for a mobile phone and a computer, accessing the internet is a capability and not a learned skill. The following sentence demonstrates the different implications.
The following examples show how néng 能 and huì 会/會 have different implications when they are used in the same syntactic context.
When the modal verbs are used as in the examples above, degree adverbs and adverbial phrases such as hěn 很 ‘very’ or bú tài 不太 ‘not very’, can be placed before them to indicate different degrees.
In connection with physical capabilities, quantitative expressions can be added:
If néng 能 is replaced with huì 会/會 here, the sentence Wáng xiānsheng huì chī wǔ wǎn fàn 王先生会吃五碗饭/王先生會吃五碗飯 cannot be understood as ‘Mr. Wang can eat five bowls of rice’, because eating rice is not a skill. The sentence with huì can only be understood as ‘Mr. Wang will eat five bowls of rice’.
If kěyǐ 可以 is used here (Wáng xiānsheng kěyǐ chī wǔ wǎn fàn 王先生可以吃五碗饭), this can be understood in two ways: ‘Mr. Wang can eat five bowls of rice’ (because his physical condition allows him to do so) or ‘Mr. Wang is allowed to eat five bowls of rice’.
2. néng 能 and kěyǐ 可以 expressing a possibility
To ask about the possibility of someone doing something, one can use either néng 能 or kěyǐ 可以 to express ‘would it be possible…’. Answers to this kind of “possibility-requiring” question are xíng 行 and kěyǐ 可以 ‘yes’ or bù xíng 不行 ‘no’.
If you want to say, “It’s possible for someone to do something,” you can use kěyǐ 可以. In some situations, the sentence will be perceived as a suggestion.
Note that there is a special usage of néng 能: it is also commonly used to indicate that something is objectively possible or impossible when a specific situation or reason is explicitly mentioned.
In this context, néng 能 and kěyǐ 可以 are not interchangeable. Otherwise, the sentence Wǒ shēng bìng le, bù kěyǐ shuō huà 我生病了，不可以说话/我生病了，不可以說話 will be understood as ‘I am not allowed to talk because I am ill’.
3. néng 能 and kěyǐ 可以 expressing permission
Néng 能 and kěyǐ 可以 can both be used in a situation when permission is being requested. Answers to this kind of “permission-requiring” question are xíng 行 and kěyǐ 可以 ‘yes’ or bù xíng 不行 ‘no’.
Note that when asking for permission, only kěyǐ 可以 can be used in a tag question.
For prohibitions, the negation forms bù néng 不能 ‘cannot’ and bù kěyǐ 不可以 ‘not allowed’ can both be used; bù kěyǐ 不可以 sounds a bit less formal.