Although yídìng一定 and bìxū必须 can both mean ‘must’, yídìng一定 is better to be understood as ‘definitely’. There are some circumstances that they are not exchangeable.
1. When the we want to guarantee something, we should use yídìng一定, for example,
Wǒ yídìng nǔlì xuéxí
I definitely hard study
‘I will definitely work hard.’
2. When we want to estimate something, we should use yídìng一定, for example,
Míngtiān yídìng xiàyǔ
Tomorrow definitely rain
‘It will definitely rain tomorrow.’
