Obligation
Must
When there is a strong necessity(obligation) to do
something like a set of rules to follow or duties to perform, we use the
modal verb must.
E.g.,
You must only use blue ink pen for the exam.
Must not - Negative
Must not implies that there is a strong obligation
not to do something.
E.g.,
You must not eat during class.
There are two things you must note,
1. Negative form of the must is not no obligation.
It's an obligation of what not to do.
E.g.,
You must not eat during class.
Must not expresses the obligation that you must not
eat during class.
2. Must neither has a past form nor any attachments
to indicate the past. In some cases of modals, to refer to the past, you use
modal+have+past participle. But if you use that with must, it becomes a
prediction sentence.
E.g.,
He must have stolen the jewelry.
It's not an obligation. It's a prediction/deduction
sentence.
So, to indicate the opposite of obligation, and to express an obligation that was implied in the past, we use have to and don't
have to.
Have to
Have to is the same as must. It
imposes a strong obligation.
E.g.,
I have to attend the meeting. I am an important board
member.
Don’t have to - Negative
Don't have to means there is no obligation to do
things. It’s up to the person's decision.
E.g.,
You don't have to clean the car. It will rain
today. You can just leave it in the rain.
Had to – Past
Had to expresses obligation that was implied in
the past.
E.g.,
I had to wear hand me down clothes when j was a kid
since my family couldn't afford new ones.
Advice and Suggestion
Should
Should is used when you suggest or advise someone
to do something. Suggestions and advice are weaker obligations.
E.g.,
You should go see a doctor. You are burning.
Should not – Negative.
E.g.,
You shouldn't wander in the streets at night. It's
dangerous. Go home.
Should have/shouldn’t have - Past
The past form expresses regret for not following a
suggestion or advice.
E.g.,
You did not pass the entrance exam. You should've
studied harder.
I shouldn't have left school so late. I
missed the train.
Lesson plan
Worksheet
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