Modal Verbs of Obligation

Modal verbs of obligation help us express necessity and requirements. In this lesson we'll focus on the three key modal verbs of obligation:

must
have to
need to

We'll explore their meanings, grammar, and how to use them correctly.

Must, Have To & Need To

Must

This modal verb expresses a strong obligation, usually based on personal feelings of necessity. For example:

I must finish this report by tomorrow.
She must call her mother back.
They must attend the meeting.

It can also express strong recommendations or prohibitions, such as those based on a country's laws, which if not followed will result in some kind of punishment.

You mustn't use your phone while driving.
Employees must follow safety regulations at all times.
All visitors must show ID at the entrance.

These are the forms of must for positive, negative and questions.

Forms of Modal Verb 'Must'

Note that must has no past tense. To express past obligation, we use had to. For example, "I had to work late yesterday" not "I must work late yesterday".

You might see must have + past participle to indicate the past, but this is related to deduction rather than obligation. For example:

He's soaking wet. He must have forgotten to take his umbrella.

Have To

Compared to must, the modal verb have to suggests an obligation imposed by external factors, such as a rule, or something necessary due to circumstances, rather than the speaker's personal feeling.

Also, unlike must, it can be used in different tenses:

I had to finish the project last week.
She has to attend the conference today.
We will have to submit the report next month.

These are the various tenses, forms and structure of the modal verb have to.

Tenses and Forms of Modal Verb 'Have To'

Need To

The modal verb need to expresses a necessity or requirement, often related to a specific situation. It differs from have to in that it tends to relate to necessity based on personal opinion or a particular situation rather than something external.

I needed to borrow some money.
She needs to find a new apartment.
They will need to make a decision soon.

Tenses and Forms of Modal Verb 'Need To'

Key Differences Between 'Have To' and 'Need To'

As the differences between 'have to' and 'need to' can be quite subtle, it's useful to summarise the key points.

Key Differences

External vs. Internal: "Have to" often implies an external source of obligation (rules, laws, or someone else’s requirements)."Need to" tends to express internal necessity or personal reasons for doing something.
Formality: "Have to" can be more formal and is used in both spoken and written contexts, especially when discussing laws or external rules. "Need to" can be slightly less formal and is often used in everyday, personal contexts.

Example Comparison:

Have to: "You have to wear a helmet when riding a bike." (This is a law or rule.)
Need to: "You need to wear a helmet to be safe when riding a bike." (This is a personal recommendation or necessity based on the person's particular view point)

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