It’s Only the Word Only
In Mandarin the word zhǐ 只 (only) usually comes before a verb. In English it is common to say “only” directly before a number, but as a rule of thumb you need to use a verb in Chinese.
Imagine you see a cute baby and ask tā jǐ suì 他几岁？ (how old is he?). In English the mother might reply “he is only one year old”, but in Chinese they would say tā zhǐ yǒu yí suì 他只有一岁 (he only have one year old).
a cute baby boy
Another example is if someone asks nǐ de hàn yǔ bān yǒu duō shǎo xué shēng 你的汉语班有多少学生？ (how many students are there in your Chinese class?). In English you can reply “only twelve” but in Mandarin you would say zhǐ yǒu shí èr gè 只有十二个 (only have twelve)
The other word for only
There is another word for “only” (cái 才) that can appear directly before a number. In the baby example above the mother could reply tā cái yí suì 他才一岁 (he only one year old). Similarly, when someone asks how many people there are in you Chinese class you can say cái shí èr gè 才十二个 (only twelve).
Although you can use cái directly before a number, it is also perfectly correct to use it in front of a verb. If you’re talking about how long you’ve studies Chinese you can say wǒ zhǐ xué le yí gè xué qī 我只学了一个学期 or you can say wǒ cái xué le yí gè xué qī 我才学了一个学期 and they both mean “I have only studies one semester”.
There is no difference in meaning between zhǐ 只 and cái 才 in these examples. They can be used interchangeably to emphasise that a number is low. However, if you want to use “only” as a way of excluding other possibilities or options then you need to use zhǐ.
For example, if your friend asks nǐ xiǎng chī shén me 你想吃什么？ (what do you want to eat?) you can reply wǒ zhǐ xiǎng chī diǎn miàn bāo 我只想吃点面包 (I just want to eat some bread) meaning you don’t want to eat other things. Because we’re not emphasising that the number is low we can’t say wǒ cái xiǎng chī diǎn miàn bāo.
Or maybe you’re back at school after the summer holiday and someone asks fang jià qù nǎ lǐ wán le 放假去哪里玩了？ (where did you go during the holiday?). In this situation you might say wǒ zhǐ qù le shàng hǎi yí tàng 我只去了上海一趟 (I only went to Shanghai).
a trip to Shanghai
Again, this sentence is emphasising that you didn’t go to anywhere else other than Shanghai so you can’t say wǒ cái qù le shàng hǎi yí tang. However, you can use cái if want to emphasise a low number and say you only went to one city. wǒ cái qù le yí gè chéng shì, shàng hǎi 我才去了一个城市，上海 (I only went to one city, Shanghai).
Other uses of cái
You can use cái in sentences that don’t indicate a low number, and the above sentence wǒ cái qù le shàng hǎi yí tang is not grammatically incorrect. However, the meaning is completely different to the meaning of “only” discussed here. For more information on this usage click here.