Will is used to express promise, willingness, or voluntary actions that will take place in the future. It is also used to make predictions. Be going to is also used to express predictions, but not willingness or volition.
Examples:
| Promise: | The four brothers promised that they will come out of their shells every day. |
| Voluntary Action: | The man will weave a cradle for the four brothers. |
| Prediction: | The Lao people think the shells of the four brothers will heal all kinds of sickness. The four brothers are going to be rulers of the Lao people. |
Would is the past form of the modal verb will. It is commonly used to create conditional verb forms and to indicate repetition in the past.
Examples:
| Conditional: | If the old couple would remain childless, they would have no one to care of them. |
| Past habits: | When the old couple first received their unexpected gift, they would always marvel at their good fortune. |
Must is used to express certainty, necessity, or strong recommendation. Sometimes though, speakers prefer to use the more flexible term have to.
The modal phrase must not is used to express prohibition. However, because of the sharpness of the word, speakers often substitute it with the words should not or ought not.
Examples:
| Certainty: | The four young lads must say good-bye to live in their shells again. |
| Necessity: | The four young lads must live in their shells. |
| Strong Recommendation: | The old couple must follow the young men's instructions. |
| Prohibition: | The young men must not be allowed to turn into complete human beings before the twelfth lunar month. |
Ought to is used to ask for advice or make recommendations and express assumptions or expectations and strong probability.
Examples:
| Recommendation: | The couple ought to stop asking the young lads to take human form. |
| Expectation: | The young men ought to be understood. |
| Probability: | Living with the four young men, the couple ought to improve their way of living. |
Ought not is used to ask for advice against doing something, but should not or had better not are often preferred by American speakers. In using ought not the particle to is omitted.
Examples:
To ask for advice against (something): The couple ought not reveal their secret.