Notes On Some More Modals - CBSE Class 9 English Grammar
Modal verbs as Synonyms
    
The modal verbs SHOULD and OUGHT TO are close synonyms; WILL and WOULD are synonyms in some types of polite requests. It is sometimes difficult for language learners to choose among synonymous modal verbs. Always choose a more general synonym out of a group of synonyms or a synonym with a more straightforward meaning.

For example, MAY, MIGHT, COULD are synonyms in the meaning "possibility". MAY has two other meanings; COULD has four other meanings, two of which are rather difficult to distinguish from "possibility" without a clear context. Only MIGHT has just one meaning: possibility. The choice should be obvious – use MIGHT or MAY for "possibility".


Modal verbs as phrasal modals
Examples: be able to, be going to, be supposed to, have to, have got to, ought to, used to.
With these phrasal modals, the simple form of a verb follows to:

She's able to help us.
They are going to help us.
We are supposed to help them.

Summary

Modal verbs as Synonyms
    
The modal verbs SHOULD and OUGHT TO are close synonyms; WILL and WOULD are synonyms in some types of polite requests. It is sometimes difficult for language learners to choose among synonymous modal verbs. Always choose a more general synonym out of a group of synonyms or a synonym with a more straightforward meaning.

For example, MAY, MIGHT, COULD are synonyms in the meaning "possibility". MAY has two other meanings; COULD has four other meanings, two of which are rather difficult to distinguish from "possibility" without a clear context. Only MIGHT has just one meaning: possibility. The choice should be obvious – use MIGHT or MAY for "possibility".


Modal verbs as phrasal modals
Examples: be able to, be going to, be supposed to, have to, have got to, ought to, used to.
With these phrasal modals, the simple form of a verb follows to:

She's able to help us.
They are going to help us.
We are supposed to help them.

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