V + 得 / 不得: allowed, not allowed (吃不得, 动不得)
【五35】补语 / complements
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
V + 不得 says that something must not (or had better not) be done, because it would be dangerous, harmful or improper: 这种药吃不得 (this medicine must not be taken), 腰动不得 (the back must not be moved). It is a prohibition or a warning, not an inability. The positive V + 得 (这个菜吃得: this dish is safe to eat) is rarer and appears mostly in the question “allowed or not?”: 吃得还是吃不得？
What is talked about comes first, as the topic: 重东西拿不得, not 拿不得重东西. To stress the warning, people often put 可 before the verb: 这个你可碰不得. Only a handful of basic verbs fit this mould: 动、吃、碰、拿、说、笑、骂、打. It is spoken Chinese, the voice of the doctor, the parent, the proverb.
Do not mix it up with ability: V不了 (吃不了: too much for me, I can't manage) and V不起 (买不起: can't afford). 这么多菜我吃不了, but 这种菜我吃不得: I am not allowed to eat it.
Examples
这种药饭前吃不得。
Zhè zhǒng yào fàn qián chī bu de.You must not take this medicine before a meal.
你的腰刚受伤，现在动不得。
Nǐ de yāo gāng shòushāng, xiànzài dòng bu de.Your back has only just been hurt: you must not move it for now.
这是他的伤心事，你可碰不得。
Zhè shì tā de shāngxīn shì, nǐ kě pèng bu de.That is a sore subject for him, so don't you touch it.
这话在老板面前可说不得。
Zhè huà zài lǎobǎn miànqián kě shuō bu de.You must never say that in front of the boss.
这两种菜，哪一种吃得，哪一种吃不得？
Zhè liǎng zhǒng cài, nǎ yì zhǒng chī de, nǎ yì zhǒng chī bu de?Of these two dishes, which can be eaten and which must be avoided?
这么多菜我吃不了，那盘鱼我更吃不得，我对鱼过敏。
Zhème duō cài wǒ chī bu liǎo, nà pán yú wǒ gèng chī bu de, wǒ duì yú guòmǐn.I can't finish all these dishes, and that fish I must not eat at all: I am allergic to it.
Common mistakes
✗ 这么多菜我吃不得 → ✓ 这么多菜我吃不了 (“I can't manage it all” is 不了; 吃不得 means “I mustn't eat it”).
✗ 吃不得这种药 → ✓ 这种药吃不得 (the thing talked about comes first, as the topic).
✗ 我买不得这件衣服 → ✓ 我买不起这件衣服 (“can't afford” is 不起).