Modal verbs: must and have to

Modal verbs are auxiliary verbs (i.e. verbs that cannot be used as main verbs, acting thus as complements) with many different uses. They can be used to express obligation, ability or possibility as well as ask for permission or make requests. Today we are going to focus on obligation with must and have to. Even if these verbs possess similar meanings they differ in certain aspects as we will see below.

Must is used to express obligation, necessity or prohibition. Let’s see some examples:

• We must leave this place now, I think there is a gas leak (obligation)
• She must get up at 7 o’clock if she wants to start work at 8 (necessity)
• You mustn’t smoke in the metro (prohibition)

Additionally, must can be used to express assumption or probability, for example:

• He must be at work now because he is not answering the phone (probability).
• I guess it must be difficult to complete studies on aeronautics (assumption)

Have to can also be used to express obligation or necessity, but not prohibition in its negative form.

• We have to pay all our taxes before the end of June (obligation)
• You have to be at least 6 feet tall to become a policeman (necessity).

As you can see, must and have to possess very similar meanings, even though they differ slightly. Generally, must refers to obligations between people or self-imposed, whereas have to refers to external, social or legal obligations.

In its negative form, however, have to has a completely different meaning, since don’t have to refers to the complete lack of obligation. For instance:

• We are meeting at the pub after work but you don’t have to come if you don’t want to
• The extra lessons are optional, we don’t have to attend if we don’t want to

We hope you find this useful, now try with the following exercises:

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