需：need 要：ask for 必：must 须：have to 需要[v.]：It means something/somebody is needed by somebody. 我需要一份工作。 必须[adv.]：It means somebody have to do something. 你明天必须交作业。 必需[v.]：Something which can not be lacked of, objectively needed. 食物是生活所必需的。 造句：空气是生存所必需的，每个人都需要呼吸新鲜的空气，所以我们必须保护环境。

需要、必須 they have same usage sometimes, but not always. for example: there are some letters you must reply 這裡有些信件你必須要回復 or 這裡有些信件需要你回復 both sentences here are correct, but use them in this sentence: I need your love 我需要你的愛 ≠ 我必須你得愛 which you can find out, in first sentence they both mean "must" or "need to", in the second sentence, 需要 have a meaning of need, but 必須 here still mean "must" or "need to" it will make whole sentence really confusing. 必需 mean something you need as an adjective water are indispensable in desert. 水在沙漠是必需的(不可或缺) or 水在沙漠是需要的 需要 can also be an adjective when you put a 的 behind it and at this sentence 必需的 is stronger than 需要的 basically 需要、必須 have same usage in certain sentence, and 需要的、必需的 are similar, but 必需的 are stronger than 需要的. 必須 and 必需 can not be replaced by each other let me make a conclusion: use 需要 and 必須 to modify verb will have same meaning use 需要的 and 必需的 as an adjective will have similar meaning because 必須 is used to modify verb, 必需 is as an adjective, so they are different. this is a good question, even native Chinese will get confused sometimes: ) it's a disaster when I put the answer in comment column.. it become unreadable and I can't not delete it : (

谢谢你

需要、必須 they have same usage sometime, but not always. for example: there are some letters you must reply 這裡有些信件你必須要回復 or 這裡有些信件需要你回復 both sentences here are correct but use them in this sentence: I need your love 我需要你的愛 ≠ 我必須你得愛 which you can find out, in first sentence they both mean "must" or "need to" in the second sentence, 需要 have a meaning of need, but 必須 here still mean "must" or "need to" it will make whole sentence really confusing. 必需 mean something you need as an adjective water are indispensable in desert. 水在沙漠是必需的(不可或缺) or 水在沙漠是需要的 需要 can also be an adjective when you put a 的 behind it and at this sentence 必需的 is stronger than 需要的 basically 需要、必須 have same usage in certain sentence, and 需要的、必需的 are similar, but 必需的 are stronger than 需要的. 必須 and 必需 can not be replaced by each other let me make a conclusion use 需要 and 必須 to modify verb will have same meaning use 需要的 and 必需的 as adjective will have similar meaning because 必須 use to modify verb, 必需 as an adjective so they are different. this is a good question, even native Chinese will get confused sometimes: )