為了準時趕上公車，我不得不邊吃早餐邊穿衣服 為了有好的考試成績，我不得不刪掉手機遊戲讓自己能專心 手實在是太酸，我不得不先停下來休息 我必須趕緊把工作做完，才能玩遊戲 他必須先把工具準備好，才能開始做蛋糕 我需要一個剪刀，謝謝 他需要你的讚美

不得不 = no choice, implies being forced 人, 不得不吃, 然而就成仙了. Man has no choice but to eat, otherwise he will die (become an immortal, a euphemism) 須 = must, should, followed by a verb or adverb 人須要吃.... Man must eat, otherwise... 必 = essentially, nexessary, certainly. So 必須 = certainly must, so it acts as a sort of emphasis. 人必要吃. Man certainly have to eat... So it is the same as 人須要吃. 需 = need, require, demand, followed by a noun 人需要食物.... Man needs food, otherwise... In English, all these Chinese differences can be eliminated because the words used are interchangeable without much difference in understanding. This sort of translation is for language lawyers. 得 = have, to gain something, obtain etc.

飛機出故障了，這樣下去會墜毀， 我們不得不減輕重量　：　there is no other choices. , it forcing us do this 沒有其他選擇了，我們被逼這麼做 我們必須減輕重量　：　there is no other choices , we should determine to do this 沒有其他選擇了，我們該下定決心這麼做 我們得(dei3)減輕重量　：we should determine to do this , unless there is another idea 除非有什麼其他主意，不然我們就該這麼做了 我們需要減輕重量　：　Maybe we can refuse to do this , but if we don't , it put us to a hard position.　我們或許可以不這麼做，但將使我們處於不利境地 我們最好減輕重量　：　Maybe we have other choise , but this is a easy way / this is the best way ＂得(dei3),需要,最好＂，all are used to express advise , while ＂不得不，必須＂are only used to give a notice . ＂需要,最好＂，all are used to express in your feeling "I should" ; ＂得(dei3)，必須＂，is used to express in your feeling "I must" ; ＂不得不＂，is used to express "must" just in a cold fact . 比起＂不得不＂只能表示被迫，＂必須＂卻可以表示主動意願，在＂被迫這麼做＂與＂主動這麼做＂之間，可以有主動來做的選擇餘地。 ＂不得不＂means forced to this, while ＂必須＂means choose between " be forced to do" or "do by one's own " 比起＂得(dei3)＂表示一種催使It make me to do，＂需要＂卻可以表示這種催使可以被無視It need me 比起＂得(dei3)＂表示一種堅定的主動意願，I must do it，＂需要＂卻可以表示只是輕微的意願，I want do it。 好比I must save the girl 與 I need to save the girl ，前者是＂得(dei3＂，後者是＂需要＂

不得不=have to do something ,usually unwilling but to force to do sth 得= need to do sth 必須=must'.必須后面接句子或动词 需要=need,可以接名词或者句子

得 必须 需要 are same meaning just like need must and have to 不得不is like you have no choice and you must do it sorry for my bad english ^.^