How to distinguish and use negative prefixes in, im, il, ir, un, dis, et. for different cases.
Words that take dis- as a negative prefix may begin with a vowel or a consonant.
Words that take il- as a negative prefix always begin with the letter l.
Words that take im- as a negative prefix always begin with the letter m or p.
Words that take in- as a negative prefix can begin with a vowel (except i or u) or a consonant.
Note: There are many words that begin with in- that are not words with a negative prefix. For example:
word does not mean incline not cline
indulge not dulge
insist not sistin
voke not voke
Words that take ir- as a negative prefix always begin with the letter r.
Words that take non- as a negative prefix may begin with a vowel or a consonant.
Words that take un- as a negative prefix may begin with a vowel or consonant.
Remember, not all words that appear to have a negative prefix are negative.
word does not mean
alike not like
discuss not cuss
universe not iverse
illuminate not luminate
important not portant
involve not volve
irrigate not rigate
I gained access to this list at THIRUMAVALAVAN's blog. Very good rules on negative prefixes from Agnes Sonik below..