Modal Verbs

In this lesson you’ll learn how to use modal verb constructions and their meanings.

Lesson: Must, When & When Not to Use It

When Not To Use Must: For Obligations

obligation, noun /ˌɒblɪˈɡeɪʃən/
Something you do because it’s your duty or because you feel you have to.

You must do your homework.

Don’t use must for obligations if you’re a non-native speaker because:

  1. We almost don’t use it at all for that ourselves,
  2. It’s very difficult to understand when you can use it instead of have to, and
  3. In those cases when you can use it, it’s never wrong to use have to instead.

Hopefully you can see when you put these reasons together, that using have to for obligations is a better and safer strategy.

When To Use Must: For Deductions

deduction, noun dɪˈdʌkʃən/
The process of deciding something is true using available information.

There’s a long line outside that restaurant. The food must be good there.
That girl’s crying. She must be sad.

Sherlock Holmes was a master at making deductions. Use must when you, like him, make observations and draw conclusions from them.

Deductions about things things in the past

Must is a modal verb that doesn’t have a past form. Modal verbs without past forms are put into the past by putting the verb that comes after them in the perfect.

She must be sad. -> She must have been sad.
She got a perfect score on her exam. She must have studied a lot.

Have to for deductions

Have to can be used for deductions too, but aren’t as often.

That book is a bestseller. It has to be good.

And in the past:

It’s strange that she hasn’t gotten here yet. Something has to have happened.

Deductions made using have to are a little bit weaker than ones made with must. Speaker sound a little bit less sure about their conclusions.