💡 What Are “Obligation” and “Necessity”?

We use modal verbs like must, have to, need to, and their negatives to talk about things that are necessary or not necessary. For example, rules, duties, or responsibilities.
We can also use them to talk about permission in the present or past.

🔹 1. Obligation in the Present

We use must, have to, and need to when something is necessary or important to do.

Structure	Example	Meaning
must + base verb	I must finish this project today.	Strong personal obligation
have to + base verb	I have to work on Saturday.	External rule or schedule
need to + base verb	You need to wear a helmet.	It’s necessary or required

🧠 Tip:

• must = personal obligation (you feel it)
• have to = external obligation (someone else decides it)

I must study more. (I want to)
I have to study more. (My teacher said so)

🔹 2. NO Necessity in the Present

We use don’t have to or don’t need to when something is not necessary.

Structure	Example	Meaning
don’t have to + base verb	You don’t have to come if you’re busy.	It’s optional
don’t need to + base verb	She doesn’t need to wear a uniform.	It’s not required

🧠 Note:
They mean the same, but don’t have to is more common in everyday English.

🔹 3. Prohibition (Something Is Not Allowed)

We use mustn’t (must not) to talk about rules or strong advice — when something is not allowed.

Structure	Example	Meaning
mustn’t + base verb	You mustn’t park here.	It’s forbidden
be not allowed to + base form	You are not allowed to park here.	No permission

🧠 Tip:
❌ Don’t confuse mustn’t (not allowed) with don’t have to (not necessary).

You mustn’t smoke here. → It’s against the rules.
You don’t have to smoke. → It’s your choice.

🔹 4. Obligation and Necessity in the Past

We can’t use must for the past. Instead, we use had to or needed to.

Structure	Example	Meaning
had to + base verb	I had to finish my homework last night.	It was necessary in the past
needed to + base verb	We needed to buy tickets early.	It was necessary

For the negative, we use didn’t have to or didn’t need to (something was not necessary).

We didn’t have to go to school yesterday.
She didn’t need to call — I was already there.

🔹 5. Permission (Present and Past)

We can use be allowed to or could to talk about permission.

Time	Structure	Example	Meaning
Present	am / is / are allowed to/can	Students are allowed to use phones at break. Students can use their phones at break.	It’s permitted now
Past	was / were allowed to	We were allowed to stay up late.	It was permitted
Past (general ability)	could	When I was a child, I could go out alone.	I had permission

🔍 Summary Table

Meaning	Present	Past
Obligation	must / have to / need to	had to / needed to
Not necessary	don’t have to / don’t need to	didn’t have to / didn’t need to
Prohibition	mustn’t	—
Permission	am / is / are allowed to/ can	was / were allowed to / could

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Modals of Obligation and Possibility

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Let’s Practice

Practice 1

Practice 2

Practice 3

Correct the grammar mistakes

Speaking