Obligation, need, possibility and probability

1. Obligation
Must is an instruction or command; that is why we see it on notices
   Dogs must be kept on a lead.
   Cars must not be parked here
Have (got) to says that cirumstances oblige (v- buộc) you to do something. Often, the two meanings overlap and there will be a choice of how to express the oblication, but not always.

_ The company is obliged to give a refund if the tour is cancelled.
_ You will be liable (adj- có bổ phận) to pay tax if you work.
_ The bank robbers forced (v- ép, áp đặt) him at gunpoint (n- chĩa súng) to open the safe.
_ We had no choice/alternative but to sell our house; we owed the bank $100,000
_ The death sentence is mandatory for drug-smuggling in some countries.
_ Was sport compulsory (adj- buộc, phổ cập)/ obligatory (adj- bắt buộc) at your school? No, it was optional at mine.
_ I am exempt (adj- được miễn) from tax as I'm a student. (free from obligation)

The negative of must and have (got) to are formed with need and have to, when we mean something is not necessary/ not obligatory.

_ They were losing $1 million a year, so the company was obliged/ forced to close down/ had to close down.
_ You don't have to buy travel insurance, it is optional.
_ You can hire a video camera, but you have to pay a deposit.
_ We'll have to sell the house, I'm afraid we have no choice/alternative (n), otherwise we'll go/be bankrupt (v- phá sản, vỡ nợ).
_ You're over 50, you're exempt from military service/ not obliged to do military service.

2. Scalse of probability: 'cannot happen' to 'has to happen'
impossible (adj- ko thể xẩy ra)
unlikely
possible
probable
certain
inevitable (adj- chắc chắn xẩy ra)

_ Most people usually suffer from a lack of time or of money.
_ Filling out a tax return is obligatory once a year in many countries.
_ Death is certainly inevitable for all of us.
_ A videophone in every home is quite possible by 2025.
_ Rain in the Amazon forest within eight days is highly likely!
_ A human being living to 250 is absolutely imposible.
_ Most people feel they are in need of more time and money, and millions of people in the world are in need of food and a decent home.
_ A third world war? Very possible if we continue to build nuclear weapons.