Expressing Obligation

By Rob Lane


In the last post, I wrote about how ability is expressed using modal verbs and phrases. In this article, I will give an overview obligation. Obligation may be described as pressure on a person to do something or not to do something.

There are strong obligations such as rules and necessities, and weak obligations such as advice. Obligations may be internal, from the speaker’s body or mind, and external obligations such as regulations.

In this post, we will see only the most common forms used to express standard obligation. Other, more advanced forms will be looked at in future posts. There are a number of important rules that you should take special care to understand and practice.


Strong Obligation

In present, need to, must and have to are all used to express strong obligation. All three have equal strength. Often learners have difficulties with must and have to and often see little difference between the two. In brief, the rule is that must is used for internal obligations, and have to is used for external obligations.

My tooth is sore. I must go to the dentist.

To travel, you have to carry a passport.

Although this is the guiding rule, there are so many exceptions to it that it is a weak rule. The rule should be applied. You should be prepared for plenty of examples that go against it.

The negative forms of these verbs are also of interest.  Compare these examples:

You must not bring food into the class.

You don’t have to bring food into the class.

The first prohibits bringing food into class. The second says that it is not necessary for you to bring food in but you may.

Learners should take special care with structures such as must have done and should have done. These forms will be looked at in a later post.


Weak Obligation

In present, weak obligation is often described in grammar books as the right/ correct thing to do. Weak obligations may come from tradition, custom or culture and may be seen as advice.

Should and ought to are the most common verbs used. There is no difference between the two.

Often, learners are unfamiliar with ought to as this may be difficult to hear in conversations because it is contracted.

Our friend is unwell. We should visit him. (It is a nice thing to do.)

You ought to prepare your bags the night before your flight. (It is a good idea.)


Obligation in Past

Obligation in past in much more simple: use only needed to or had to. Had to is most common. There is no difference in strength or internal/ external in the past.

As mentioned earlier there are a number of other structures such as: be supposed to, must have done, should have done etc. These structures are not used to standard obligation and will be looked at in future posts.


You Should

Pay special attention to the strong and weak, internal and external rules, and the differences in present and past.

Write a dialogue between two people comparing their obligations in the past with today.