Silent and invisible letters in English
Silent letters constitute a notorious phenomenon in English: in wréstle, for example, only four out of the seven letters are actually sounded (*résl), and there can be strings of them in place names, exemplified by the trio Léicester, Glóucester, Worcester (*Léster *Glóster *Wùster). (The accents show stress and pronunciation, see English phonemes.)
But redundant letters can serve to distinguish between words that sound the same:
knôw knowledge = nô negative
knót tie = nót negative
wráp parcel = ráp knock, talk
wrîte read = rîght correct = rîte ritual
chéck verify = BrE chéque money
Typical silent letters are b finally after m or before final t (-mb -bt); g or k initially before n (gn-, kn-); gh finally or before final t ( -gh -ght); l after à and before final f or m (-lf -lm); n finally after m (-mn).
List
Silent A is found in: hëard lëarn Múrray = Mòray nébulaê; BrE words ending in -ary: sécondary díctionary; and all examples from Latin of aê: nébulaê (in the latter case, the American spelling omits the 'a')
B: thúmb dúmb númb clîmb límb débt dòubt súbtle
C: indîct Tûcsón Connécticut blancmange (*bləmónzh); after s before a front vowel: scêne scîence effervésce