Grammar Chaos: Mastering Modals—Knowing When to Use Must, Should, Have To, and Ought To

If you have a habit of interchanging must, should, and have to, then you ought to read on.

Although the four abovementioned words/phrases are often harmlessly interchanged, there actually is a specific use for each one. In fact, many linguists, grammarians, professors, scholars, and general enthusiasts of the English language have tried to differentiate them. To an extent, they’ve succeeded.

We all know how they’re used—they precede an action that is intended to be a necessity or an obligation. Collectively, they are called modal verbs. Must and should are what we call modal auxiliary verbs. Have to and ought to, on the other hand, are called modal paraphrases.

What they differ in is the source of the obligation, as well as strength. Must and have to are used for stronger obligations when enforcing orders. Often they are used by some outside form of authority whether it’s a person (boss, parents) or phenomenon (fate, weather). Should and ought to are intended for giving advice. These obligations often come from one’s conscience or as a suggestion from people.

Here are some examples to help illustrate their differences.

   Sandra, you must go home now. – implies an order

   Sandra, you should go home now. – implies a suggestion

Let’s try the other set.

   I have to finish this paper by tonight. – implies urgency and obligation

   I ought to finish this paper by tonight. – implies a thought, but there’s no sense of urgency

When a sentence is in the affirmative, all these modal verbs mean the same. However, for the stronger modals, the of modals change when the sentence is negative.

   Sandra, you must go home now. – implies an order

   Sandra, you have to go home now. – implies an order

As opposed to:

   Sandra, you must not go home now. – implies that doing the action is potentially dangerous

   Sandra, you do not have to go home now. – implies that the obligation to do the action is nonexistent

For the softer modals, the only difference is that should is more widely used than ought to.

Beware that must, have to, should, and ought to could imply different meanings depending on the context they are used in. However, for modals, what matters is that you know to differentiate when you’re giving/being given an order or a suggestion.

So should you remember this little lesson? Well, you must.

Grammar can be a little chaotic, but hey, we’re here to help you make sense of it. Watch out for more Grammar Chaos articles! Drop us a line if you have some questions or suggestions.

Sources:

• Grammar: teaching the modals 'ought to', 'should', 'must' and 'have to'.
• What is the difference between “have to”, “must”, and “should”? [duplicate]

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