These words are known as modal verbs obligation and are also called modal auxiliary verbs. They are used to express an obligation, that is to say, something which you are obliged to do. Each word can be used to express varying degrees of obligation in both the positive and the negative sense.
These verbs are typically used in conjunction with an infinitive. That means that they will usually appear in the form of ‘have to’ plus ‘infinitive’, ‘should’ plus ‘infinitive’ and ‘must’ plus ‘infinitive’.
Form of should, must and have to
Should, must and have to: positive
|
I You We They |
should / must / have to |
dance (base form of the verb)
|
| He / She / It | should / must / has to |
Should, must and have to: negative
|
I You He / She / It We They |
shouldn't / mustn't | dance (base form of the verb) |
|
I You We They |
don't have to |
dance (base form of the verb) |
| He / She / It | doesn't have to |
Should, must and have to: question
|
Should/must |
I You He / She / It We They |
help (base form of the verb)? |
| Do |
I you we they |
have to | help (base form of the verb)? |
| Does | he / she / it |
Uses of should, must and have to:
Should
In the present tense, ‘should’ can be used in order to signify a milder form of obligation and/or advice. The negative version is made by placing a ‘not’ after the word: ‘should not’, indicating a milder form of negative obligation or piece of advice.
You should always put aside some of your earnings in a savings account. (Positive, present)
You should consider donating to charity when you can. (Positive, present)
You should have filled the kettle to the top when making a cup of tea. (Positive, past)
I should not have run when the ground was wet. (Negative, past)
You shouldn’t use that machinery without the proper protection. (Negative, present)
We should not try to remove a red wine stain with white wine. (Negative, present)
Have to
In the present tense, ‘have to’ denotes a strong obligation, likely from an external perspective. The positive is simply ‘have to’ and the negative is made by using ‘don’t’ before it: ‘don’t have to’. The negative indicates no obligation.
People have to look before crossing the road. (Positive, present)
We have to find out how to make these delicious cakes before the chef leaves. (Positive, present)
You didn’t have to go to school during the summer. (Positive, past)
I don’t have to go to work today because it’s a weekend. (Negative, present)
You don’t have to worry too much about combing your hair today. (Negative, present)
The dogs did not have to worry about politics, they just chased cats. (Negative, past)
Must
In the present tense, ‘must’ is also used to denote a strong obligation but this is typically from a speaker’s perspective. To form the negative version, the speaker places ‘not’ after the word: ‘must not’, indicating that there is a negative obligation, i.e. something is forbidden.
I must think about what I have learned at school today. (Positive, present)
You must go home before your dinner gets too cold. (Positive, present)
I had to post that letter or I would have missed the delivery. (Positive, past)
I must not tell lies about my mathematics homework. (Negative, present)
You didn’t have to turn the heating on. (Negative, past)
We must not smoke cigarettes as it leads to ill health. (Negative, present)
Should, must and have to:be careful!
There is a difference between the use of ‘must not’ and ‘do not have to’ or ‘don’t have to’. The former suggests that the action is not allowed or that it is a bad idea, while the latter suggests that an action is allowable even if it is not necessary.
When using in the past tense, ‘have to’ becomes ‘had to’ and ‘didn’t have to’. For those who wish to use ‘must’ in a past tense, then the typical pattern is to use ‘had to’ and ‘didn’t have to’ as it will be able to provide the same meaning.
In order to use ‘should’ in the past tense, the speaker is able to use ‘should have’ followed by the past participle or, in the case of negative versions, ‘should not have’ plus the past participle.
In any instance, the use of ‘not’ can be contracted to ‘n’t’ and attached to the end of a word, for example: ‘shouldn’t’ instead of ‘should not’ and ‘mustn’t’ rather than ‘must not’.
The phrase ‘must have done’ is not included with the modal verb obligations and is considered instead to be a modal verb of speculation or deduction. It does not refer to an obligation which has happened in the past
It is important to remember that there will be no ‘-s’ ending in the 3rd person singular.
