Expressing One's Own Obligations, with
～なければならない
I have to ...

The ～なければならない (or, more formally, ～なければなりません) structure is one that needs to be learnt as a chunk. It simply means "have to ..."
A note to help you make sense of this extremely useful phrase:
  • It's made up of two parts: ～なければ ("if I don't ～") plus ならない／なりません (things won't go well for me... loosely translated!)​
  • It's generally used to talk about one's own personal obligations, rather than someone else's. 
An example to whet your appetite:
ぼくは、日本語のしけんに　ごうかくしたいので、毎日べんきょうしなければなりません。
  • I want to pass my Japanese exam, so I have to study every day. (Tom!)
Forming the structure:
To form the first half (the ～なければ bit):
  • Begin with a verb in the plain form negative. E.g. しない (I don't do); やらない (I don't do); 行かない (I don't go)
  • Remove the final い and replace with ければ
  • So しない becomes しな＋ければ (if + I don't do); やらない becomes やらな＋ければ (if + I don't do); 行かない becomes 行かな＋ければ (if + I don't go). (Obviously, you don't need the plus sign in between...)​
To form the second half (the ならない or なりません bit).
  • Simply add ならない or なりません, depending on the level of formality you want.
  • Which gives you しなければなりません・やらなければなりません (both mean "I have to do"); 行かなければなりません ("I have to go")
Some more examples to get excited about!
家にかえったら、いぬのえさを　やらなければなりません。
  • When I get home, I have to feed the dog. (えさをやる = feed an animal)
雨がふっていても、いぬのさんぽを　しなければならない。
  • Even if it's raining, I have to walk the dog. 
You can use it in the past tense, too.
きょ年の夏休み、りょうしんの仕事を　てつだわなければなりませんでした。
  • Last summer holidays, I had to help with my parents' work. 
日本に行ったとき、日本語しか　話さなければならなかったです。
  • When I went to Japan, I had to speak only Japanese. 
食事がきらいでも、ぜんぶ食べなければなりませんでした。
  • Even if I didn't like the food, I had to eat everything. 
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PRACTICE "NAKEREBA NARIMASEN" ON QUIZLET