Prefixes (voorzetsels) change the meaning of a word. We do the same in Dutch:
legaal > illegaal
gewild > ongewild
regelmatig > onregelmatig
alfabeet > analfabeet
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Are there any rules?
Not really.
You can only study prefixes by studying the words they belong to. Sometimes it is easy, because you already know a lot of them. Sometimes you have to look them up in a dictionary.
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Are there any rules?
There are minor rules for spelling though.
ir- is always followed by an r irregular
im- is always followed by an m or p immaterial
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Also remember
Not every word with a prefix is negative or opposite: