Prepositions are difficult in any language, and Chinese is no exception. One pair of Chinese prepositions that seems to trip up native English speakers quite a lot is 对 (duì) and 跟 (gēn). They don’t seem to map to any prepositions in English, but they come up a lot in Chinese grammar and can control the meaning of verb phrases in quite significant ways.
The main difference to focus on is that 对 usually implies a one way relationship, whilst 跟 is used for two-way relationships. In other words, 对 expresses that the action of the verb goes in one direction (from subject to object), whereas 跟 expresses that it goes in both directions.
It might help to rigidly think of 对 as meaning “to”, and 跟 as meaning “with”. Sometimes this will produce strange sentences if you translate into English, but this can often be a good way to build up a stronger sense of Chinese grammar.
对 and 跟 with 说 (shuō)
A very common use of 跟 and 对 is with the 说 (shuō) - “to speak”. 对 and 跟 are often interchangeable when used in sentences about telling people things, or saying things to people. For example:
我跟他说了。
Wǒ gēn tā shuōle.
I told him.
我对他说了。
Wǒ duì tā shuōle.
I told him.
A lot of people would say that there is no real difference between 跟 and 对 in this situation, but we tend to believe in the rule of ‘no true similes’. That means that even if native speakers can’t quite say what it is, there will always be some kind of difference between two different words, at least in certain contexts. See below for more detail on that.
In general though, you can assume that using 对 or 跟 with 说 doesn’t really make a significant difference. More examples:
他跟我说他要去旅行。
Tā gēn wǒ shuō tā yào qù lǚxíng.
He told me that he was going travelling.
医生对你说了什么？
Yīshēng duì nǐ shuōle shénme?
What did the doctor say to you?
老师跟我说我必须努力一点。
Lǎoshī gēn wǒ shuō wǒ bìxū nǔlì yīdiǎn.
My teacher told me I have to work harder.
In each of these examples, you could swap 跟 and 对 and there wouldn’t be much difference in the meaning. Note that these sentences are all about telling or saying things to people. Those are the situations when 跟 and 对 are interchangeable. Otherwise, you’ve got to pick more carefully.
When the preposition should be 跟 and not 对
As mentioned above, 跟 is often used when the action of the verb is two-directional, or there is a two-way relationship between the subject and object. More intuitively, 跟 is also used to express things being done together or with other people. Here we’ll look at two situations involving 跟 that can be tricky to get used to.
Using 跟 for discussions and explanations
We explained above how 跟 and 对 can be interchangeable when used in sentences about “telling” and “saying to”. However, there are many situations that involve speaking where the choice of 跟 and 对 does make a big difference.
One of those is sentences about discussions, explanations or revelations. In other words, if there is a focus on a conversation or two-way exchange, the sentence is unlikely to use 对. Have a look at some examples:
我想跟您商量一下。
Wǒ xiǎng gēn nín shāngliáng yīxià.
I'd like to discuss it with you.
我跟你讲……
Wǒ gēn nǐ jiǎng……
Let me explain...
我有一件事想跟你说说。
Wǒ yǒuyī jiàn shì xiǎng gēn nǐ shuō shuō.
There's something I'd like to talk to you about.
你提前跟我说就没问题。
Nǐ tíqián gēn wǒ shuō jiù méi wèntí.
If you tell me beforehand it'll be fine.
In these examples, if you swapped 跟 for 对, the sentence would become quite strange or just plain wrong. For example “我对你讲” has a sense of “I am talking to you and you are listening” - it becomes a one-way action rather than the two-way action implied by 跟.
If you say “我有一件事想对你说” rather than the sentence above, it becomes “I want to tell you something”, but focuses much more on the one-way action of telling, rather than any conversation that might surround it.
The sentence with 商量 simply can’t use 对, otherwise it would be like saying “I’d like to discuss something to you”. A discussion is a two-way action, so it only makes sense with 跟.
Using 跟 for comparisons
Another common use of 跟 as a preposition is to make comparisons between two things (when used like this it’s the same as 和). This can confuse native English speakers who might expect to use 对, as “to” is often used to make comparisons in English. In Chinese, though, you say that “A and B are different” or “A is the same with B”. Have a look at some examples:
他跟我很不一样。
Tā gēn wǒ hěn bù yīyàng.
He's very different to me.
我的看法跟你的一样。
Wǒ de kànfǎ gēn nǐ de yīyàng.
My opinion is the same as yours.
她跟以前不一样了。
Tā gēn yǐqián bù yīyàngle.
She's different to how she was before.
英式英语跟澳式英语很像。
Yīngshì Yīngyǔ gēn àoshì Yīngyǔ hěn xiàng.
British English is very similar to Australian English.
Remember that Chinese prefers to express similarities and differences as a two-way relationship (“these two things”), whereas English tends to express them as a one-way relationship (“this thing to that thing”, “this thing from that thing”). Some more examples to reinforce this point:
她跟你一样高。
Tā gēn nǐ yīyàng gāo.
She's as tall as you.
德国的面包跟法国的不一样。
Déguó de miànbāo gēn Fàguó de bù yīyàng.
German bread is different to French bread.
请把这首诗跟昨天学的那首比较一下。
Qǐng bǎ zhè shǒu shī gēn zuótiān xué dì nà shǒu bǐjiào yīxià.
Please compare this poem to the one we studied yesterday.
Notice how all of these comparisons can be expressed using 跟 in Chinese (they could also be expressed with 和), whereas English has a few different prepositions for the different situations.
When it should be 对 and not 跟
In some situations, the preposition has to be 对 and not 跟, otherwise the sentence would be incorrect. These are situations with a one-way action or one-way relationship from subject to object. Have a look at some examples:
她对数字的记忆力很强。
Tā duì shùzì de jìyìlì hěn qiáng.
She has a good memory for numbers.
对这个话题来说，我没有想法。
Duì zhège huàtí lái shuō, wǒ méiyǒu xiǎngfǎ.
I don't have any opinion on this topic.
那个人对我们公司的影响很大。
Nàgè rén duì wǒmen gōngsī de yǐngxiǎng hěn dà.
That person had a big effect on our company.
我想对你们的帮助表示感谢。
Wǒ xiǎng duì nǐmen de bāngzhù biǎoshì gǎnxiè.
I'd like to thank you for your help.
("I'd like to express thanks for your help.")
Notice how each of these sentences is about a one-way effect or action. The object is affected by the subject, but not the other way round (or at least the sentence doesn’t mention any action in the other direction).
When using 对 or 跟 changes the meaning
There are situations where using either 对 or 跟 would be grammatically valid, but the meaning is significantly different. For example:
我跟他很好。
Wǒ gēn tā hěn hǎo.
I'm on good terms with him.
我对他很好。
Wǒ duì tā hěn hǎo.
I treat him very well.
As you can see, the difference between a one-directional action and a two-directional one is very different. Another example:
政治家对听众讲话。
Zhèngzhì jiā duì tīngzhòng jiǎnghuà.
The politician spoke to the audience.
政治家跟听众讲话。
Zhèngzhì jiā gēn tīngzhòng jiǎnghuà.
The politician spoke with the audience.
The difference here isn’t so significant, but it’s still clear. In the first sentence with 对, only the politician is doing the talking. When 跟 is used, it seems that a two-way discussion is going on.
We said above that when used with 说, 对 and 跟 are pretty much the same. However, if you change the verb a little bit, there is a more obvious difference:
老板每天跟员工说话。
Lǎobǎn měitiān gēn yuángōng shuōhuà.
The boss speaks with the employees every day.
老板每天对员工说话。
Lǎobǎn měitiān duì yuángōng shuōhuà.
The boss speaks to the employees every day.
There is a clear-cut difference between the meaning of those sentences in Chinese and English, and you can see how the use of 跟 (“with”) vs 对 (“to”) makes this happen.
Summary of 跟 and 对 as prepositions
Very similar / the same when used with 说 to talk about “telling” or “saying to”.
跟
Can be thought of as meaning “with”.
Usually implies a two-way action.
Can often imply a continuous action or exchange.
对
Can be thought of as meaning “to”.
Usually implies a one-way action.