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

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!

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.