你必须8:00之前赶到公司，否则扣你奖金。 你需要在8:00之前到达公司，要不然会被扣奖金的。 see, when we say 需要，it's more euphemistic than 必须。 the difference between 必需 and 必须 is that, the former include the later. 必需＝必须需要 举例：生活必需品 但不能说：生活必须品

我需要休息，必须的!

谢谢大家！ It's much clearer to me now, thanks!

必須= have to 我必須要走了= I have to go. 需要= need 我需要一些水= I need some water. 希望能幫助到你! Have a lovely day x

I think your question is what is the difference between必须 and 需要 when they're both used to say that you have to do something. In simpler words, the difference between "必须do sth" and "需要do sth". In this case, 必须 sounds more demanding. For example: At the police station, the officer would say: 你必须 fill up the form, which means that is a must, you have no choice. On the other hand, when you're buying a...say...insurance policy, they would say politely that 你需要fill up the form. I suppose this is what you said about 'use them naturally'. Hope it helps.