obligation / need / possibility

obligation : 義務

• [形]liable : 法的な義務の意味も含む
  You will be liable for any damage when you rent a car
• [形]obliged
  The company is obliged to give a refund if the tour is cancelled
• [形]forced : 強制（make的な使い方）
  The bank robbers forced him at gunpoint to open the safe.
• have no choice/alternative but : するしかない
  We had no choice/alternative but to sell our house.
• [形]mandatory : 必須の(法律、ルール、約束などによって強制される性質のもの,免れないみたいな？)
  The death sentence is mandatory for drug-smuggling in some countries.
• [形]compulsory / obligatory : 強制
  Was sport compulsory/obligatory at your school?

Need : 必要

• in need of : ~を必要として(needs/wantsよりフォーマルな言い方)
  This plant is in need of water.
• need for : 〜を必要とする (we feel a need)
  There’s need for more discussion on the matter.
• an absolute necessity : 必ず必要
  A car is an absolute necessity if you live in the countryside and there is no public transport.
• requirements : 必要条件
  What are the requirements for a Master’s course in English?

possibilities : 可能性

• Is there any chance/possibility you’ll be free next week?
• [形]inevitable : 必然
  A rise in oil prices is inevitable as oil supplies run out across the world.
• It is doubtful whether : かどうかは怪しい
  It is doubtful whether the economy will improve next year.
• It is probable that : たぶん〜だろう
  It is probable that the next government will change the tax laws.
• certainty : 確実
  Most scientists believe that climate change is no longer just a possibility but is now an absolute certainty.