129. Differences between Necessity Verbs


Different shades of necessity meaning are expressed by different English modal verbs


Necessity verbs suggest a pressure to carry out the action of another verb placed straight after. Unless the second verb is passive, the person or thing under pressure is likely to be the subject of the sentence, like this:

(a) Vehicle owners MUST HAVE insurance.

Necessity verbs are quite numerous in English, reflecting a wide variety of subtypes of necessity. Many of the verbs are listed elsewhere within this blog in the post 65. Verbs that Mean “Must” or “Can”. Here I wish to examine those of a more grammatical nature that are sometimes called “modal”, such as BE TO, HAVE TO, MUST, NEED, OUGHT TO, SHALL and SHOULD, which seem to differ from each other in particularly subtle and confusing ways.

The meanings of these verbs are all commonly explained in mainstream grammar descriptions. What I hope to offer is a number of novel interpretations that might make the meaning differences slightly easier to understand. Other posts within this blog that deal with modal verbs include 51. Making Concessions with “May”60. Purpose Sentences with “For” and 119. BE Before a “to” Verb.



The necessity verbs listed above can be divided into two groups according to the cause of the necessity they express. In one group, the cause occurs before the moment when the necessity verb is spoken, so that the sentence as a whole is just reporting it, while in the other it is created at that moment. Here is a summary of the possibilities:


The details of this classification are as follows.

1. BE TO

This verb is extensively analysed in a previous post (119. BE Before a “to” Verb). Necessity is only one of its many meanings. Mostly it is necessity created before the moment of utterance. The commonest type is arrangements, as in this example:

(b) A meeting of finance ministers is to be held in Brussels.

This would normally mean that the necessity of the meeting was created earlier by people making an arrangement, which the speaker is merely reporting. No necessity is being created through the words being spoken.

Another previously-created necessity that BE TO can report is that resulting from formal regulations, which MUST also commonly expresses (see below). The following sentence might suggest this if spoken to the staff of an organization:

(c) Visitors are to report to Reception.

Using are to here instead of must makes the necessity sound less dictatorial, perhaps by suggesting it is based on communal agreement.

BE TO can also create necessity at the moment of its use, mainly of the command type. Sentence (c) could be a command – again more gentle than with MUST – if addressed to the public.

To make the negative of BE TO, simply add not; for the past tense, use past forms of BE.



This verb nearly always expresses necessity. The primary use is to report a previously-established necessity without suggesting how it may have come about. Consider this:

(d) The children cannot attend because they have to do homework.

It is not clear here what previously caused the necessity: it may have been a school command or regulation, or a parental command, or the children themselves driven by a belief in homework. Using must instead of have to would more strongly suggest that an order had been given, or a regulation was being followed, or the speaker agreed strongly with the necessity.

The other main use of HAVE TO establishes rather than reports a necessity. The particular kind seems to be exhortation – urging the subject of the verb to behave in a way that the speaker believes to be necessary. The following sentence could be understood to have this meaning, although it could also be taken to report an established necessity:

(e) People showing these symptoms have to visit their doctor.

NEED TO and MUST can also exhort (see below), but add extra meaning (danger-avoidance and urgency respectively). Interestingly, if (e) is not reporting and not addressed to anybody, it becomes more wish-like.

A less formal equivalent of HAVE TO in all its uses is HAVE GOT TO. The past tense of HAVE TO is had to. The negative form is a little tricky. If the meaning is “compelled not to”, one can use HAVE TO NOT, but it is rare, the preference being for mustn’t or shouldn’t or a paraphrase like BE NOT ALLOWED TO. Using DO NOT HAVE TO expresses the absence, not the presence of necessity (= “allowed to”).


3. NEED (TO)

This verb nearly always suggests that the necessity is caused by danger. Its function can be just information-giving, without expecting the addressee to take any action, or action-seeking. An example of the first might be:

(f) Salmon need to spawn near river sources.

The danger implied here to be the cause of the necessity is easily recognized, of course, as reproductive failure. Have to could also be used. Although it would be vaguer about the reason for the necessity, this would not greatly affect understanding because the reader’s general knowledge would compensate. Must is possible too. It would suggest some kind of law – natural law perhaps.

The action-seeking uses of NEED include exhorting (strongly encouraging) and advising (presenting a recommended option). The relevant danger is more likely to be associated with the time of speaking. Sentence (e) above would be a stronger exhortation with need to instead of have to. Even (f) could in a suitable context be understood as an exhortation.

The past tense form of NEED is needed. The negative is tricky like that of HAVE TO: DO NOT NEED TO and NEED NOT remove the necessity, NEED TO NOT keeps it. For more information about using to with NEED, see 148. Infinitive Verbs without “to”.


4. MUST (1)

This verb is common for both reporting and creating necessity. The reporting use normally involves rules, regulations, laws or moral beliefs. Other people’s commands can also be reported. Examples are:

(g) You must have a ticket to travel.

(h) You must be kind to your neighbour.

Spoken by an authority figure, (g) would imply that a legal requirement was being stated, (h) either a moral law or a command from a higher authority. Using must in (f) instead of need would perhaps imply natural law.

The other main use of MUST, creating necessity at the time of speaking, can have different purposes. A common one is to give one’s own strong command (stronger than with BE TO or SHALL). Sometimes this is directly addressed to the people who are expected to obey. Sentence (c) above, replacing are to with must, could have this function, as could (g). At other times the command is often in a formal written document, e.g.:

(i) The claimant must notify the company within three days.

Another non-reporting use of MUST is to highlight particularly important instructions. Instructions differ from commands in being cooperative with their addressees rather than coercive (see 128. Imperative Verbs in Formal Writing). They will mostly use alternatives to MUST, like imperative or ordinary passive verb forms.

Thirdly, non-reporting MUST can express very strong exhortation or advice. Sentence (h) could be understood in this way, as could the following:

(j) People showing these symptoms must consult their doctor.

The past tense form of MUST with all these meanings is had to. The negative uses not.


5. MUST (2)

This is the logical reasoning use, as in sentences like the following:

(k) x squared is 9 so x must be 3.

The necessity here is the absence of any other possibility. It can be a previously-established fact or created at the moment of speaking. The past tense form is again had to. The negative, however, is CANNOT.



This verb will not usually express necessity after I or we, and quite often will not after other subjects either. Its main necessity use is the statement of regulations in legal documents, rather like MUST though perhaps with slightly weaker force. It would hence be possible instead of MUST in (i) above.

Sometimes SHALL reports rather than creates regulations, as in this possible public notice:

(m) Bins shall be emptied every Friday.

The use of shall instead of must is perhaps because the regulation, unlike that in (g), does not have to be obeyed by the people being addressed (the public do not empty their bins themselves).

The ability of SHALL to express necessity explains why it is incorrect in non-necessity statements like the following, where a decision is being communicated:

(l) Jones will play at right back.

There is no past tense equivalent of necessity-showing SHALL. One might instead say had to or was/were to. The negative uses not.



The necessity expressed by these synonymous verbs is usually of a reduced kind, implying that their subject has more choice. As a result, the general usage is advice-giving. However, the suggestion of choice means that should in particular is also often used in commands and instructions merely as a more polite-sounding alternative to must.

There is no past tense of should or ought to; one must use needed or BE + advisable. Beware of using should have: it suggests that an advisable behaviour failed to happen. The negative uses not.


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