Silent letters
Letters a word carries without sounding them. A speller working by ear leaves them out, and sometimes adds one where none belongs.
The rule
A silent letter is spelled and not said, so a spelling that leaves it out sounds exactly right. That is what makes these misses hard to catch by reading the word aloud.
Many silent letters are sounds English stopped making while the spelling stayed put: the K of knife and know, the W of write, the GH of night and drought.
Others sit in clusters English does not finish saying: the G of gnome, the N after M in autumn, column, condemn and solemn, the H of rhythm and rhyme.
Some come back in a related word, which is the most reliable way to remember them: autumn has autumnal, column has columnist, condemn has condemnation, solemn has solemnity, and sign has signature.
The other direction
Knowing that silent letters exist can also put one where it does not belong. That is the miss in psycophant and juggernaught: a familiar silent cluster, the PS of psychology or the GH of naught, laid over a word that does not have it.
Where it stops holding
No pattern predicts every silent letter. Plenty belong to a single word, like the S of island and aisle or the B of debt, and those are learned one at a time.
Background
Where most of these letters came from, layer by layer, is on where English spelling comes from.
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