不免 (bù miǎn) and 未免 (wèi miǎn) are both adverbs that are used to indicate that something is inevitable or cannot be avoided. However, there is a subtle difference in their meanings.
不免 (bù miǎn) means "inevitably" or "unavoidably," and it is used to indicate that something is certain to happen or cannot be avoided. Here is the structure when using 不免:objective reason,+不免+result
未免 (wèi miǎn) means "unavoidably" or "unnecessarily," and it is used to indicate that something is unnecessary or excessive. You use it when you can’t agree to something. Here is the structure when using 未免:(scenario, can be omitted in the sentence),+未免+judgement
Here are some examples to illustrate the difference between the two, with pinyin provided for the example sentences:
不免 (bù miǎn)
* 这种天气下出门,不免有些冷。 (zhè zhǒng tiānqì xià chūmén, bù miǎn yǒuxiē lěng) (It is inevitably a little cold when going out in this kind of weather.)
* 他的职业生涯结束了,不免有些伤感。 (tā de zhíyè shēngyá jiéshù le, bù miǎn yǒuxiē shāngǎn) (His career has ended, and it is inevitably a little sad.)
未免 (wèi miǎn)
* 这道题的难度未免有点大。 (zhè dào tí de nándù wèi miǎn yǒudiǎn dà) (The difficulty of this problem is unnecessarily large.)
* 他的表演未免有些夸张。 (tā de biǎoyǎn wèi miǎn yǒuxiē kuāzhāng) (His performance is unnecessarily exaggerated.)
* 你这么做,未免太过分。 (nǐ zhème zuò, wèi miǎn tài guòfèn) (What you are doing is unnecessarily extreme.)