Other members have already answered your question. Just for information, all below words have similar meaning. "必须“， ”得“， ”要“， ”一定要“ （ put "一定“ in front of "要” for emphasizing your motivation). You can replace those word of your own choice. Example ============ 1. I have to study for exams. 我要复习考试了 为了考试我得学习了(了= you act right now ) 为了考试我得学习 ( you may act anytime in the future ) put "为了...." at the beginning will emphasize your purpose( to indicate it is really important) another example with "了” at the end I have to study for passing the exams为了考试合格，我必须复习了 2. I have to cook for my boyfriend. 我要煮饭给男朋友吃了/我得给男朋友做饭 3. I have to study for 3 hours today. 今天我要学习3个小时 / 今天我得学三个小时 ”为了“ means ' reason.... for something / someone '. Please note, if "了“ is put at the end of a sentence , it usually indicate past tense. But here, it just indicate "change of status". ( it indicate you will act right now, no delay ) for sentence 1 and 2, it's very natural to add 了 at the end. But not for the third one, as it is about a plan, not a change of action/status. Hope it helps.

为了考试我得学习。 我得给男朋友做饭。 今天我得学三个小时。

1. I have to study for exams. 我 “必须” 学习，” 为“了考试。 more nature (change the order)-- 为了考试，我必须学习。 2. I have to cook for my boyfriend. 我“必须”“为”我的男朋友做饭。more nature--- 我“要”为我的男朋友做饭。 3. I have to study for 3 hours today. 今天我“‘必须”学习三个小时。【no need "为"in this sentence。】 "Have to":必须 “for something” ： 为...

" to have to do something " : 要做… The meaning of " for something " depends on the context. 1. I have to study for exams. 为了通过考试，我要学习。 2. I have to cook for my boyfriend. 我要煮东西给我男朋友吃。 3. I have to study for 3 hours today. 我今天要花3个小时学习。