Prefixes and suffixes: -dis, -un, -im, -in, -ir; -ful, -ness, -less, -ion, -y, -a

English as a Second Language English Vocabulary, Grammar and Idioms
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Hello!I want to learn about prefixes and suffixes.Is there any way of deciding which additional we must use.For example, -dis,-un,-im,-in,-ir ; -ful,-ness,-less,-ion,-y,-able etc.
We say unhappy but why can’t we say dishappy?I want to learn the rule,if there is.
Thank you very much!

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There are no rules for the most part. The main exception is ‘in-, im-, ir- and il-’, whose choice depends on the first letter of the root.
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