firstly, 必须 and 必需 are not the same. 必须 means "must" or "have to". -你必须得按时交作业。 -按时交作业是必须的！ 必需 is used to say that "it is necessary/it is a must to have something （or that something exists） （so that ....）" or "sth. is a necessity for sth./sb. （to ...）" -空气和水是每个人所必需的。 只得 = 不得不 means "have to". reluctant. you dont want but you have to. 一定要 means "must". it could be said by yourself. or imperatively. -我一定要好好学习！！ -你一定要好好学习呀！ I use 呀 in the last sentence because i think that when said to somebody imperatively, the mood of 一定 is not as strong as 必须. it's more of an expectation than an order.

必须=must 只得=have got to 一定和必须差不多

To be frank , simplely i think that the difference between Chinese and English is that we can use different Chinese words to show the same meaning , while we can use one English word to decrible the different meaning so the word above are the same meaning ,no specific difference

基本没差别咯！:例如我一定要去和我必须要去！THE same （细微差别只在说话的习惯上） 例如：我一定要去旅游 我必须要去旅游！（用必须要像有点被别人别什么东西强迫了，。。）这里必须就不常用 另个例子：我一定考第一名（表决心） 我必须考第一名（感觉有危机感，不考第一难道会被骂）哈哈 仔细体会

必需是东北的常用语，只得是书面语，平时不用，一定是另一个意思，三者并非同义，只是前两个同点而已，像你掏钱是必需的-口头，我只得打个的了-用只好更多，不过现在区别真不太大，意会即可