Look at this word: uncomfortable.
We can break it into un (prefix) comfort (base word) and able (suffix).
We pronounce this word like this:
In other words, the 'or' in the middle is silent. So if you know the pronunciation maybe you will try to spell it *'uncomftable'* This is wrong. Usually (not always I'm afraid) the spelling of the base word doesn't change even if the pronunciation changes.
The part before the base word ('un') is the "prefix".
Two things to remember about prefixes:
1. A prefix usually changes the meaning of the word (makes it negative or opposite, for example).
2. The full prefix is added to the full base word. So:
unnecessary ( 'un' + 'necessary': so double 'n')
misspelling ('mis' + 'spelling': so double 's')
illegal ('il' + 'legal': so double 'l')
disappear ('dis' + 'appear': so only one 's')
misspelling ('mis' + 'spelling': so double 's')
illegal ('il' + 'legal': so double 'l')
disappear ('dis' + 'appear': so only one 's')
This is one reason why Albrow says that English spelling is 'for the eye rather than the ear' (Albrow, K.H., The English Writing System: notes towards a description, Longman 1972).