The auxiliary verb 得 (děi) means "must." Yes, it's annoying that the same character can also be pronounced "de," but at least when you use 得 (děi), there's no ambiguity like with the word 要 (yào).
Normally, when you see 得, it's a particle and is pronounced "de." However, it can also be an auxiliary verb. In this case it's pronounced "děi" and means "must."
Subj. + 得 + [Verb Phrase]
Wǒmen děi zǒu le.
We have to leave now.
Shíjiān bù zǎo le, wǒ děi huíjiā le.
It's getting late. I have to go home.
Tā shì nǐ dìdi, nǐ děi bāng tā.
He is your younger brother. You must help him.
Bù hǎoyìsi, nǐ děi mǎshàng líkāi.
Excuse me, but you have to leave here right now.
Zài xuéxiào jiù děi tīng lǎoshī de huà.
You have to listen to the teacher at school.
Nǐ děi xiān fù qián zài jìnqù.
You need to pay before you go in.
Shì nǐ de cuò, nǐ děi dàoqiàn.
It's your fault. You must apologize.
Tā méiyǒu jīngyàn, nǐ děi jiāojiao tā.
He doesn't have experience. You must teach him.
Míngtiān qù miànshì, nǐ děi chuān de zhèngshì yīdiǎn.
You must wear something more formal for tomorrow's job interview.
Nǐ bìng de tài lìhai le, děi qù kàn yīshēng.
You are really sick. You must go see a doctor.
[adapted from AllSet Learning Chinese Grammar Wiki, Creative Commons License BY-NC-SA 3.0]
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