别有用心 have ulterior motives
Explanation
指在表面上或言语上表现友好或热心，而实际上有其他不可告人的目的或打算。
To have an ulterior motive; to have a hidden agenda.
Origin Story
话说唐朝时期，有一位名叫李白的诗人，他以其才华横溢，诗歌豪迈而闻名天下。有一天，李白在洛阳城里闲逛，遇到一位名叫王昌龄的诗人。王昌龄是当时诗坛上的新秀，他为了结交李白，便主动上前与李白攀谈，并热情地邀请李白去他家中做客。李白为人豪爽，对王昌龄的邀请也欣然接受了。 到了王昌龄家中，王昌龄热情款待李白，并拿出自己珍藏的美酒佳肴招待李白。李白酒量甚好，也不客气，喝得兴致高昂。席间，王昌龄拿出一张纸，对李白说：“李兄，我写了一首诗，请您指教一下。”李白接过诗歌，仔细阅读，然后对王昌龄说：“王兄，你的这首诗写得很好，很有才气，只是有一处地方需要修改一下。” 王昌龄听了，谦虚地说：“请李兄指教。” 李白指着诗歌中的几句，说：“这几句诗，虽然写得很好，但是感觉有点过于直白，缺乏一些含蓄的韵味。如果把这几个字改成这样……” 王昌龄听了李白的建议，觉得很有道理，于是就按李白的建议修改了诗歌。 酒过三巡，菜过五味，李白和王昌龄谈得非常投机，他们互相欣赏对方的才华，谈论诗歌，畅谈人生，两人都觉得相见恨晚。 后来，李白离开洛阳，回到长安。他回到长安后，便将王昌龄的诗歌传给了一些朋友，并大肆夸赞王昌龄的才华。王昌龄听到李白对自己的夸奖，心里十分高兴。 然而，李白并没有真正地欣赏王昌龄的才华，他只是想利用王昌龄的才华来抬高自己的身价。因为王昌龄当时的名气远不如李白，所以李白将王昌龄的诗歌传给朋友，并大肆夸赞王昌龄，目的是想通过王昌龄来提升自己，以便获得更多的关注和赞赏。 李白这种做法，就叫做“别有用心”。
It is said that during the Tang Dynasty, there was a poet named Li Bai who was known for his brilliant talent and powerful poetry. One day, Li Bai was strolling through the city of Luoyang when he met a poet named Wang Changling. Wang Changling was a rising star in the poetry world, and to get to know Li Bai, he approached him and chatted with him, inviting Li Bai to his home as a guest. Li Bai, known for his generosity, readily accepted the invitation. At Wang Changling's house, Li Bai was warmly welcomed and entertained with his best wines and delicacies. Li Bai, known for his good thirst, did not let himself be outdone and drank copiously and merrily. During the meal, Wang Changling took out a piece of paper and said to Li Bai, “Brother Li, I have written a poem that I would like to read to you.” Li Bai took the poem, read it carefully, and then said to Wang Changling, “Brother Wang, your poem is very good, full of talent, but there is one place I would improve.” Wang Changling, humbly, said, “Please, Brother Li, enlighten me.” Li Bai pointed to a few lines in the poem and said, “These lines are well written, but they sound a bit too direct and lack a touch of subtlety. If you were to change these few words like this…” Wang Changling found Li Bai's suggestion sensible and revised the poem accordingly. After the third cup of wine and the fifth course of food, Li Bai and Wang Changling had a very stimulating conversation. They admired each other’s talent, discussed poetry, and talked about life. They both felt they had met too late. Later, Li Bai left Luoyang and returned to Chang'an. After his return to Chang'an, he showed Wang Changling's poem to some of his friends and praised Wang Changling's talent in the highest terms. Wang Changling was very pleased with Li Bai’s praise. But in reality, Li Bai did not truly admire Wang Changling's talent. He simply wanted to use Wang Changling's talent to boost his own reputation. Since Wang Changling was not as famous as Li Bai at the time, Li Bai wanted to enhance his own profile and receive more attention and appreciation by showing off Wang Changling's poems and praising him. Li Bai's actions are an example of “Bié Yǒu Yòng Xīn”.
Usage
这个成语一般用来形容人的行为或言语，意指其表面上的表现与真实目的不符，暗藏着不可告人的企图。
This idiom is generally used to describe someone's actions or words, meaning that their outward appearance and true purpose do not match, and they have a hidden agenda.
Examples
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他表面上答应，实际上别有用心。
tā biǎo miàn shàng dā yìng, shí jì shàng bié yǒu yòng xīn.
He agreed on the surface, but actually had ulterior motives.
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我们不能被他的花言巧语所蒙蔽，他别有用心。
wǒ men bù néng bèi tā de huā yán qiǎo yǔ suǒ méng bì, tā bié yǒu yòng xīn.
We cannot be fooled by his sweet words, he has ulterior motives.
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他总是对别人说些好话，其实别有用心。
tā zǒng shì duì bié rén shuō xiē hǎo huà, qí shí bié yǒu yòng xīn.
He always says nice things to others, but in fact has ulterior motives.
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他向我示好，不知道他别有用心。
tā xiàng wǒ shì hǎo, bù zhī dào tā bié yǒu yòng xīn.
He is nice to me, I don't know what he is up to.