非得 … 不可 (absolutely must)

You have to see a doctor

• Your friend has had a fever for three days but refuses to see a doctor.
• you你nǐto burn烧shāo(degree marker)得deso much这么zhè me(used to describe sb or sth…厉害lì hài，must非得fēi děigo去qùhospital医院yī yuàn(absolutely)不可bù kě。 — "With a fever this bad, you absolutely have to go to hospital."

must非得fēi děi + verb (+ (absolutely)不可bù kě) = must, have to

• must非得fēi děi (fēi děi) = must, absolutely have to; (absolutely)不可bù kě / won't do不行bù xíng can close the sentence for extra force.
• In speech, must非得fēi děi alone often works: you你nǐmust非得fēi děitoday今天jīn tiāngo去qù(yes/no ?)吗ma？

you你nǐmust非得fēi děitoday今天jīn tiāngo去qù吗ma？

Necessity and insistence

• Necessity: this这份zhè fènreport报告bào gàomust非得fēi děitoday今天jīn tiānbe handed in交jiāo(absolutely)不可bù kě。 — "This report absolutely must be handed in today."
• Insistence: he / him他tāmust非得fēi děioneself自己zì jǐto make做zuò，who谁shéito help帮忙bāng mángall / both都dōudon't不要bú yào。 — "He insists on doing it himself and won't take help from anyone."

this这份zhè fènreport报告bào gàomust非得fēi děitoday今天jīn tiānbe handed in交jiāo(absolutely)不可bù kě。

Read it fēi děi

must非得fēi děi versus (must)非fēi…(absolutely)不可bù kě

• (must)非fēi…(absolutely)不可bù kě puts the verb in the middle: (must)非fēigo去qù(absolutely)不可bù kě.
• must非得fēi děi…((absolutely)不可bù kě) is the spoken variant with (degree marker)得de: must非得fēi děigo去qù / must非得fēi děigo去qù(absolutely)不可bù kě. Both are emphatic.

I我wǒ(must)非fēigo去qù(must)不可bù kě。

Recap

Read the full reference: HSK 6 grammar — Sentence patterns.