In order to distinguish the differences between bì xū (必須), bì xū (必需), and xū yào (需要), we have to examine the definitions of xū (須) and xū (需).
Though both words can be roughly translated as "need" in English, their usages are quite distinctive in Chinese: xū (須) means "must" or "have to," while xū (需) indicates "need" or "require."
Therefore, bì xū (必須) is an adverb expressing the notion that somebody have to do something.
→ 你明天必須交作業 (nǐ míng tiān bì xū jiāo zuò yè/ You have to hand in your homework tomorrow.)
bì xū (必需) is an adjective describing something which is indispensable or objectively needed.
→ 食物是生活所必需的 (shí wù shì shēng huó suǒ bì xū dí/ Food is indispensable in our lives.)
xū yào (需要) is a verb indicating something or somebody is needed by somebody.
→ 我需要你的愛 (wǒ xū yào nǐ dí ài/ I need your love.)
