Shall vs. Must

'Shall' and 'must' are modal verbs that may be confusing as they both refer to necessities. In this lesson, we will learn their differences.

What Is Their Main Difference?

The main difference between modal verbs 'shall' and 'must' is that 'shall' refers to the future while 'must' refers to the present and future.

'Shall' is a modal verb. Modal verbs (also known as modals) are used to give more information about the main verb. 'Shall' is used to talk about necessities and give offers. For example:

We shall carry through our difficulties.

We shall join the parade.

'Must' is a modal verb used to talk about necessities, likely events, and suggestions. Have a look:

I must leave as soon as possible.

She must be in class by now.

Similarities

Talking about Necessities

We use 'shall' and 'must' to talk about necessities.

'Shall' is used to talk about obligations and necessary actions in formal contexts. We often see 'shall' in documents and guidelines. Have a look:

All parties shall respect the court's decisions.

The final product shall be on my desk by Monday.

'Must' is also used in less formal contexts to express strong obligation. It refers to both the present and future. For instance:

We must hurry to catch the train.

The final product must be on my desk by Monday.

Negation and Question

Modal verbs are used to create negative sentences and questions.
To create negative sentences, we add 'not' to the modal verb as illustrated below:

Shall → Shall not → Shan't
Must → Must not → Mustn't

Take a look at these examples:

We shall carry through our fears. → We shall not carry through our fears.

You must look after your brothers. → You must not look after your brothers.

We can create yes/no questions and wh-questions with modal verbs.
Yes/no questions are made by putting the modal verbs at the beginning of the sentence followed by the subject and the main verb. For example:

I shall leave sooner today. → Shall I leave sooner today?

You must go. → Must you go?

To create wh-questions, we start by a wh-word such as what, when, where, who, why, and how followed by a modal verb, the subject, and the base form of the main verb. For instance:

I shall do something. → What shall I do?

They must be here. → Where must they be?

With Other Modals

We use only one modal verb in a sentence. We cannot use modal verbs with other modal verbs. Take a look at these incorrect sentences:

You ~~shall~~ ~~can~~ pass this test.

You ~~must~~ ~~will~~ deny that you saw me with the body.

Differences

Making Offers

We use 'shall' to make formal offers. In this form, we need pronouns 'I' or 'we' to accompany 'shall.' We cannot use 'must' in this context as it will convey a sense of order. For instance:

Shall I take your coat?

Shall we take the longer route?

Formality and Usage

'Shall' and 'must' are both formal when talking about necessities. However, 'shall' is way more formal than 'must,' and since 'shall' is very uncommon, it may sound pretentious and unnatural. Therefore, in normal contexts, we mainly use 'must.'

With Conditionals

'Must' is used with conditional type 1. In this type of conditionals, we are talking about a condition and the results that follow. These conditions have a high chance of occurrence. For instance:

If you want to drive the car, you must be careful.

If he is getting that much money, he must save some of it.

You may hear 'shall' in conditional type 1 statements, but this usage is mainly dedicated to dialects.

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