This page has other details about modal verbs and subtle differences between modal verbs with similar meanings.
Some modal and semi-modal verbs have identical or similar meanings. Some of the differences can be very subtle.
CAN AND BE ABLE TO
“Can” and “be able to” can be used interchangeably though “be able to” or “be unable to” is more formal than “can.”
- Many European children can speak more than one language at an early age.
- Many European childnre are able to speak more than one language at an early age.
- Could you solve the problem?
- Were you able to solve the problem?
“Can” and “could” are the only possible verb tenses to express their modalities; therefore, “be able to” is useful to express the same ideas in any verb tense. All you need to do is conjugate “be.”
For example, you cannot conjugate “can” in the present perfect tense, but you can do so with “be able to” as in the following examples:
- The workers have been unable to strike a deal with their employers.
- As observation techniques have improved in the last 100 years, astronomers have been able to see increasingly farther into space.
You can also combine “be able to” with other modal verbs, which is impossible with “can” or “could.”
Possibility + ability:
Depending on weather conditions, you may be able to see several meteorites per hour during tonight’s meteor shower.
Low likelihood + lack of ability:
If you were a woman living 100 years ago, you might not be able to survive childbirth.
Necessity + ability:
To be considered for this job, applicants must be able to speak English, Spanish, and Mandarin fluently.
Logical conclusion + past ability:
You were well prepared for the test. You must have been able to pass without any problems.
Past repetitive action + ability:
When I was younger, I used to be able to run a mile without getting very tired.
Expectation + ability:
What happened? You should have been able to pass the test without any problems!
MUST AND HAVE TO
Note. Also see “must and have to expressing obligation” below.
Verb tenses with “must” and “have to”
Although “must” can express ideas in different time frames, it exists in the present tense. To express “must” in other verb tenses, “have to” can be used.
Simple past tense:
- I had to stay up late last night because I had work to do.
- The teacher had to change exam date because school was closed during the snow storm.
Future with “will” and “going to:”
- Students will have to complete all their coursework by Friday to be eligible to graduate this semester.
- If you want to pass this course, you are going to have to work harder than you have so far.
Present perfect tense:
- I’ve never had to go to court before.
- They have already had to postpone the meeting twice this week because they were not ready.
Combining other modals with “have to”
Using “have to” allows you to use other modal verbs with the ideas expressed by “must.”
Possibility + necessity:
If Professor Ecks has already left, you may have to wait until tomorrow to talk to him.
Disapproval + necessity:
People shouldn’t have to wait for two hours to renew their driver’s licenses.
Low likelihood + necessity in the past:
I wonder why John is late. He may have had to stop somewhere on the way here.
Logical conclusion + necessity in the past:
I wonder why John is late. He must have had to stop somewhere on the way here.
MUST AND HAVE TO EXPRESSING OBLIGATION
Although “must” and “have to” are often used interchangeably, they have a subtle difference in meaning.
When they express an obligation, “must” refers to an obligation that comes from the speaker. “Have to,” on the other hand, implies that the obligation comes from someone else, for example, when referring to a rule or law.
In some cases, particularly in informal, spoken language, the difference is so subtle that you can use “must” or “have to” interchangeably.
Compare the following groups of sentences, in which the difference is made:
Obligation coming from the speaker:
- We must leave now so we’re not late.
- I must start exercising again.
- I must study harder.
- We must start early tomorrow.
Obligation coming from someone else:
- I have to work this Saturday. (My supervisor told me I have to.)
- Students in this school have to wear uniforms. (It’s a school rule.)
- My professor says I have to study harder.
- We have to review Chapter 3 for the exam tomorrow.
Note that this difference applies to “must” and “have to” expressing obligation; however, “must” also expresses other meaning, in which case this does not apply. You may then need to use “must” instead or use either modal verb interchangeably. For example,
- We’re running out of eggs. We must go to the grocery store tomorrow.
- We’re running out of eggs. We have to go to the grocery store tomorrow.
- You haven’t had anything to eat. You must be hungry.
- (“Have to” is not possible.)