We use modal verbs of obligation when we want to express a person’s obligation to do something – that is, when we want to tell somebody whether they are required or expected to do something.
To express obligation, we can use have to + infinitive, must + infinitive and should + infinitive.
The rules for modal verbs of obligation vary on several factors. First, we need to decide if we want to express a positive or negative obligation. Second, we need to factor in whether the event takes place in the present or past.
We use the rules for present, positive obligation when expressing something that a person is expected or advised to do. Our word choice varies depending on the strength of the obligation or advice.
Here are some examples:
- I have to be at work by nine. (strong obligation)
- You must remember to come directly home. (strong obligation)
- He should eat more fruits and vegetables. (mild advice)
We use the rules for present, negative obligation to express that something is not required, not allowed, or not recommended.
Here are some examples:
- I don’t have to give you my address. (not required)
- He mustn’t show his face around here. (not allowed)
- You shouldn’t bite your fingernails. (not recommended)
Note that mustn’t and don’t have to mean completely different things. The first simply means that an action is not required but still allowed. The second means that something is not allowed or strongly discouraged.
In both positive and negative cases, should is used to express milder sentiments.
We use the rules for past, positive obligation when describing something that was required in the past.
- After I broke my leg, I had to use a wheelchair.
- In the nineteenth century, women had to stay at home.
When describing a failed or missed obligation in the past, we should have + past participle.
- I should have remembered that you don’t like raspberries.
We use the rules for past, negative obligation when describing something that was not required in the past.
- We didn’t have to follow a dress code at the old office.
- I didn’t have to worry about that until you showed up.
Similar to past, positive obligations, we use shouldn’t have + past participle when describing a failed or missed obligation in the past.
- You shouldn’t have turned back there, because now we’re lost.
Note that when expressing past, negative obligation, we do not use must. Remember that must have done is a modal of probability, used to speculate on what happened in the past: “He must have forgotten to close the door all the way because there’s a draft in here.”