Couldn't be happier

[Jadoon 84]

Please try to help me.
Couldn't be happier
As may, might, can & could etc. belong to the family or group of "Modals" in grammar. Everyone learn the whole set of "modals" in order to learn the correct uses of will, would etc. Similarly what topic I should study in grammar to know about all such expressions as "Couldn't be happier". Where and how can I find such more expressions and their uses?

Thank you very much

[Gillnetter]

Please try to help me.
Couldn't be happier
As may, might, can & could etc. belong to the family or group of "Modals" in grammar. Everyone learns the whole set of "modals" in order to learn the correct uses of will, would etc (I doubt that "everyone" learns the whole set. I was never taught it). Similarly, what topic I should I study in grammar to know about all such expressions as "Couldn't be happier". Where and how can I find such more expressions and their uses?

Thank you very much

This isn't something you learn from a grammar book. Such expressions are learned from a long exposure to the language.