Could, Have To, Must

"Could" is used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used inconditional sentences as the conditional form of "can."

Examples:

• Extreme rain could cause the river to flood the city. possibility
• Nancy could ski like a pro by the age of 11. past ability
• You could see a movie or go out to dinner. suggestion
• Could I use your computer to email my boss? request
• We could go on the trip if I didn't have to work this weekend. conditional

Using "Could" in Present, Past, and Future

Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "could" behaves in different contexts.

Modal Use	Positive Forms 1. = Present   2. = Past   3. = Future	Negative Forms 1. = Present   2. = Past   3. = Future	You can also use:
could possibility	1. John could be the one who stole the money. 2. John could have been the one who stole the money. 3. John could go to jail for stealing the money.	1. Mary couldn't be the one who stole the money. 2. Mary couldn't have been the one who stole the money. 3. Mary couldn't possibly go to jail for the crime.	might, may
could conditional of can	1. If I had more time, I could travel around the world. 2. If I had had more time, I could have traveled around the world. 3. If I had more time this winter, I could travel around the world.	1. Even if I had more time, I couldn't travel around the world. 2. Even if I had had more time, I couldn't have traveled around the world. 3. Even if I had more time this winter, I couldn't travel around the world.
could suggestion	1. NO PRESENT FORM 2. You could have spent your vacation in Hawaii. 3. You could spend your vacation in Hawaii.	NO NEGATIVE FORMS
could past ability	I could run ten miles in my twenties. I could speak Chinese when I was a kid. "Could" cannot be used in positive sentences in which you describe a momentary or one-time ability. Yesterday, I could lift the couch by myself. Not Correct	I couldn't run more than a mile in my twenties. I couldn't speak Swahili. "Could" can be used in negative sentences in which you describe a momentary or one-time ability. Yesterday, I couldn't lift the couch by myself. Correct	be able to
could polite request	Could I have something to drink? Could I borrow your stapler? Requests usually refer to the near future.	Couldn't he come with us? Couldn't you help me with this for just a second? Requests usually refer to the near future.	can, may, might

REMEMBER: "Could not" vs. "Might not" "Could not" suggests that it is impossible for something to happen. "Might not" suggests you do not know if something happens. Examples: Jack might not have the key. Maybe he does not have the key. Jack could not have the key. It is impossible that he has the key.

"Have to" is used to express certainty, necessity, and obligation.

Examples:

• This answer has to be correct. certainty
• The soup has to be stirred continuously to prevent burning. necessity
• They have to leave early. obligation

Using "Have to" in Present, Past, and Future

"Have to" behaves quite irregularly in the past and the future. Study the chart below to learn how "have to" behaves in different contexts.

Use	Positive Forms 1. = Present   2. = Past   3. = Future	Negative Forms 1. = Present   2. = Past   3. = Future	You can also use:
have to certainty	1. That has to be Jerry. They said he was tall with bright red hair. 2. That has to have been the right restaurant. There were no other restaurants on the street. 3. NONE	1. SHIFT TO "MUST" That must not be Jerry. They said he has blond hair, not red hair. 2. SHIFT TO "MUST" That must not have been the right restaurant. I guess there was another one around there somewhere. 3. NONE	must, have got to
have to necessity	1. She has to read four books for this literature class. 2. She had to finish the first book before the midterm. 3. She will have to finish the other books before the final exam.	1. She doesn't have to read "Grapes of Wrath." It's optional reading for extra credit. 2. She didn't have to write a critique of "The Scarlet Letter." She had to give a presentation to her class. 3. She won't have to take any other literature classes. American Literature 101 is the only required course.	must

REMEMBER: "Do not have to" vs. "Must not"  "Do not have to" suggests that someone is not required to do something. "Must not" suggests that you are prohibited from doing something. Examples: You must not eat that. It is forbidden, it is not allowed. You don't have to eat that. You can if you want to, but it is not necessary.

Exercise

"Must" is most commonly used to express certainty. It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to." "Must not" can be used to prohibit actions, but this sounds very severe; speakers prefer to use softer modal verbs such as "should not" or "ought not" to dissuade rather than prohibit.

Examples:

• This must be the right address! certainty
• Students must pass an entrance examination to study at this school. necessity
• You must take some medicine for that cough. strong recommendation
• Jenny, you must not play in the street! prohibition

Using "Must" in Present, Past, and Future

Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "must" behaves in different contexts.

Modal Use	Positive Forms 1. = Present   2. = Past   3. = Future	Negative Forms 1. = Present   2. = Past   3. = Future	You can also use:
must certainty	1. That must be Jerry. They said he was tall with bright red hair. 2. That must have been the right restaurant. There are no other restaurants on this street. 3. NO FUTURE FORM	1. That must not be Jerry. He is supposed to have red hair. 2. That must not have been the right restaurant. I guess there is another one around here somewhere. 3. NO FUTURE FORM	have to
must not prohibition		You must not swim in that river. It's full of crocodiles. You must not forget to take your malaria medication while your are in the tropics. Prohibition usually refer to the near future.
must strong recommendation (Americans prefer the form "should.")	1. You must take some time off and get some rest. 2. SHIFT TO "SHOULD" You should have taken some time off last week to get some rest.  3. SHIFT TO "SHOULD" You should take some time off next week to get some rest.	1. You mustn't drink so much. It's not good for your health. 2. SHIFT TO "SHOULD" You shouldn't have drunk so much. That caused the accident. 3. SHIFT TO "SHOULD" You shouldn't drink at the party. You are going to be the designated driver.	should
must necessity (Americans prefer the form "have to.")	1. You must have a permit to enter the national park. 2. SHIFT TO "HAVE TO" We had to have a permit to enter the park. 3. We must get a permit to enter the park next week.	1. SHIFT TO "HAVE TO" We don't have to get a permit to enter the national park. 2. SHIFT TO "HAVE TO" We didn't have to get a permit to enter the national park. 3. SHIFT TO "HAVE TO" We won't have to get a permit to enter the national park.	have to

REMEMBER: "Must not" vs. "Do not have to" "Must not" suggests that you are prohibited from doing something. "Do not have to" suggests that someone is not required to do something. Examples: You must not eat that. It is forbidden, it is not allowed. You don't have to eat that. You can if you want to, but it is not necessary.