【明報專訊】How to spell in-, is more difficult than it sounds. There are two in- prefixes used in English. One means not [as in incurable] and the other means in [as in incorporate]. They originally came from French and Latin, not Old English. In English something complicated happened. These two prefixes got spelt several different ways, in-, im-, il-, ir- or sometimes even en-.
Which prefix spelling should one use? The general rule of thumb is to use the letter that matches the root word. The in- prefix was changed to match the first consonant of the root word. So il- is used in illegal, illiterate and illegible. The N and L sounds are both made in similar ways in the mouth. Words beginning with these sounds use the prefix in similarly; these include N as in innumerable. However th- words usually use un- as in unthinkable and unthankful. (Rarely en- is used, for words with the in- meaning in, as in enthrone and enthral.) Continuing that pattern, im- is used in front of words that begin with the similarly formed M, B and P sounds, as in immodest, imbalance and impossible. The ir- form gets used before words beginning with R, such as irregular and irrevocable. However in- isn't usually used before sounds such as sh, ch and j. These use un-; for example: unshakeable, unchecked and unjust.
Having different spellings of the same prefix has led to some spelling pairs, such as enquire and inquire.
Rather than memorise the spelling of each word on its own it is easier to note some patterns. Unfortunately these in-prefix spelling patterns get complicated. One has to know which of the two prefixes are being used, sometimes for a root word not used in English. Then these prefixes are spelt several different ways.
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