Understanding the Definition of Modal Verbs: The Meanings of Must


The Oxford English Dictionary offers the following definitions of the modal verb must:

  1. Expressing permission or possibility.
  2. Expressing an insistent demand or a firm resolve on the part of the speaker or imputed to another person: was (were) determined to, insisted that I (he, etc.) would.
  3. Expressing necessity: am (is, are) obliged or required to; have (has) to; it is necessary that (I, you, he, it, etc.) should.
  4. Expressing a fixed or certain futurity: am (is, are) fated or certain to, shall certainly or inevitably.
  5. Expressing an insistent demand or a firm resolve on the part of the speaker or imputed to another person.
  6. In the main clause of a conditional sentence, or with a condition implied but not stated, expressing hypothetical necessity or obligation.
  7. In the negative, expressing prohibition.

The modal verb must firstly expresses obligation. For example:

  • One must buy at least two drinks. (One is obligated to buy at least two drinks.)
  • You must donate money to attend. (It is obligatory that you donate money to attend.)
  • All students must pass this class before taking more. (It is obligatory that all students pass this class before taking more.)

The modal verb must secondly expresses necessity. The expression of necessity overlaps with the expression of obligation. Note also that the catenative verb have to is a synonym for the must of necessity. For examples:

  • You must have a license to kill. (It is necessary for you to have a license to kill.)
  • I must finish writing my paper tonight. (I have to finish writing my paper tonight.)
  • He must buy more cinnamon. (It is necessary that he buy more cinnamon.)

The modal verb must thirdly expresses commands including prohibitions, demands, suggestions, and permissions. The expression of commands overlaps with the expression of obligation. For example:

  • You must wash your hands. (It is commanded that you wash your hands.)
  • You must try some pumpkin pie. (It is suggested that you try some pumpkin pie.)
  • Students must never enter the third floor. (It is prohibited for students to enter the third floor.)

The modal verb must fourthly expresses deductions of certainty. For example:

  • That must be Oliver at the door. (It is certain that Oliver is at the door.)
  • The pie must need more sugar. (It is deduced that the pie needs more sugar.)
  • A loose fan belt must be the problem. (It is deduced that a loose fan belt is the problem).

As with all modal verbs, there is considerable overlap in the meanings of must.


The Oxford English Dictionary offers the following definitions of the modal verb should:

  1. Formerly past tense of shall.
  2. In general statements of what is right or becoming: = ‘ought’.
  3. In clauses expressing the purposed result of some action, or the object of a desire, intention, command, or request.
  4. In statements of a former likelihood, unlikelihood, expectation, hope, fear, etc.
  5. In statements of duty, obligation, or propriety (originally, as applicable to hypothetical conditions not regarded as real). Also, in statements of expectation, likelihood, prediction, etc.
  6. Ought according to appearances to be, presumably is. Also, ought according to expectation to be, presumably will be.
  7. In a hypothetical clause expressing a rejected supposition.
  8. In a hypothetical clause relating to the future, should takes the place of shall (indicative or subjunctive), or of the equivalent use of the present tense, when the supposition, though entertained as possible, is viewed as less likely or less welcome than some alternative.

The modal verb should firstly expresses advisability including suggestions and recommendations. For example:

  • You should buy a new car. (It is suggested that you buy a new car.)
  • They should take the interstate. (It is advised that they take the interstate.)
  • Americans should not eat so much. (It is recommended that Americans not eat so much.)

The modal verb should secondly expresses necessity and, to a lesser extent, obligation. For example:

  • People with egg allergies should avoid omelets. (It is necessary that people with egg allergies avoid omelets.
  • I should wash my hands first. (I need to wash my hands first.)
  • He should stay home if he is sick. (He is obligated to stay home if he is sick.)

The modal verb should thirdly expresses predictions and deductions. For example:

  • Espen should be in Chicago by now. (It is predicted that Espen is in Chicago now.)
  • That should be Harry pulling into the driveway. (It is deducted that Harry is pulling into the driveway.)
  • We should need only two more packages of chocolate chips. (It is predicted that we need only two more packages of chocolate chips.)

Note that there is also considerable overlap in the meanings of the modal verb should. For example, the expression of necessity in I should wash my hands first overlaps with the expression of advisability; It is both advisable and necessary that I wash my hands first.

Printable Download

For a printable reference sheet of the definitions of the modal verbs must and should, please download English Modal Verbs Definitions Reference Sheet.


This post is part of the series: Semantics and Pragmatics of English Modal Verbs

Modal verbs are difficult to define because of the because of the wide range of pragmatic uses of modal auxiliaries. This series provides some of the most frequent meanings of the nine English modals—can, could, may, might, must, shall, should, will, and would—through definitions and examples.
  1. English Modal Verbs: Can, Could, May, Might, Must, Shall, Should, Will, and Would
  2. English Modal Verbs: Can and May
  3. English Modal Verbs: Will and Shall
  4. English Modal Verbs: Could and Might
  5. English Modal Verbs: Would
  6. English Modal Verbs: Must and Should