不得了 / 不敢当: amazing or serious, I don't deserve it
【五22】【五23】固定短语 / phrases
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
不得了 stands alone as an exclamation, and the context sets the meaning: "amazing, terrific" after good news (你又考了第一名，真是不得了！) or "this is serious, a disaster" after bad news (不得了了，房间里进水了！). For the second sense it usually comes as 不得了了, with a final 了, before you say what has happened.
不敢当 is the modest, polite reply to praise, thanks or a title: "I don't deserve it". It is often said twice (不敢当，不敢当) or followed by 哪里哪里. Answering "yes, I'm very good" would come across as boastful.
Don't confuse it with the degree complement 得不得了 (HSK 3), which follows a verb or an adjective and keeps 得: 累得不得了 = "incredibly tired". Here 不得了 is a sentence on its own. Register: everyday speech; 不敢当 also works in more formal settings.
Examples
你又考了第一名，真是不得了！
Nǐ yòu kǎo le dì yī míng, zhēn shì bùdéliǎo!First place again, that's amazing!
不得了了，房间里进水了！
Bùdéliǎo le, fángjiān lǐ jìn shuǐ le!Oh no, there's water in the room!
这孩子才六岁就认识这么多字，真是不得了。
Zhè háizi cái liù suì jiù rènshi zhème duō zì, zhēn shì bùdéliǎo.This child is only six and already knows all these characters. Amazing.
“您的中文说得真好！”“不敢当，不敢当，我还在学呢。”
“Nín de Zhōngwén shuō de zhēn hǎo!” “Bùgǎndāng, bùgǎndāng, wǒ hái zài xué ne.”"Your Chinese is really good!" "You flatter me, I'm still learning."
大家都叫我“专家”，我真不敢当。
Dàjiā dōu jiào wǒ “zhuānjiā”, wǒ zhēn bùgǎndāng.Everyone calls me "the expert", but I really don't deserve the title.
Common mistakes
✗ 他累不得了 → ✓ 他累得不得了 (as a complement it needs 得).
✗ 你的中文真好！ ~ 对，我很好。 → ✓ 你的中文真好！ ~ 不敢当。 (accepting a compliment outright sounds boastful).
✗ 这件事我不得了 → 不得了 does not mean "I can't" (say 我做不了 or 我不行).