This is a list of important words to learn the pronunciation of because at least one letter is spelt but not said. It is given in approximate order of when to learn them, starting with words with silent letters which are vital even for very low-level learners and ending with useful words which even very high-level learners might not know. This site also has an article on how to teach words with silent letters and worksheets on some of the words, spellings, letter combinations and silent letters in this list.
Elementary words with silent letters/ A1-level words with silent letters
answer
autumn
ballet
biscuit
buffet
build
business
Christmas
climb/ climber
could
cupboard
hour
island
knife
know/ knows/ knew
listen
raspberry
receipt
sandwich
science/ scientist
scissors
should
Wednesday
what
when
where
which
whiskey/ whisky
white
who/ whose
why
would
wrap
wrestling
write
wrong
yoghurt
Pre-Intermediate words with silent letters/ A2-level words with silent letters
aisle
anywhere/ everywhere/ nowhere/ somewhere
bomb
built
castle
comb
exhibition
foreign/ foreigner
ghost
guarantee
guard
handkerchief
handsome
honest
iron/ ironing
knee
knit/ knitting
lamb
listener
muscle
plumber
rhyme/ rhyming
rhythm
scenery
thumb
whale
wheat
wheel/ wheelchair
whether
while
whistle
whole
wrote
Intermediate words with silent letters/ B1-level words with silent letters
acquaintance
artistically
basically
bomber
cabaret
campaign/ campaigner
chalet
column/ columnist
comically
debt
dishonest
doubt/ doubtful
exhausted/ exhausting
fascinated/ fascinating
fasten/ unfasten
gourmet
hourly
knapsack
knight/ knighthood
knock/ knocker
knot
knowledge
meanwhile
medically
mortgage
parliament
physically
politically
psychiatrist/ psychiatry
psychologist/ psychology
psychotherapy
rhino/ rhinoceros
sachet
salmon
scene
somewhat
sorbet
specifically
sword
tomb
typically
vehicle
whereabouts
whereas
whilst
whirlwind
whisper
whistle
whodunnit
wholemeal
wholesale
whom
worthwhile
wrapper
wreath
wrist
Upper Intermediate words with silent letters/ B2-level words with silent letters
acquire/ acquisition
asthma/ asthmatic
beret
bidet
breadcrumbs/ crumbs
condemn
coup
critically
cynically
damn
descend/ descent
diarrhoea/ diarrhea
dumb
elsewhere
ethically
exhaustion
exhibitor
exhilarated/ exhilarating
genetically
guardian
hasten
heir/ heirloom
honour/ honourable
hymn
isle
kneel
knickers
knob
leopard
miscellaneous
obscene/ obscenity
overwhelm/ underwhelm
playwright
pneumonia
pseudonym
psychic
reign/ reigning
rendezvous
rhetoric/ rhetorical
scent/ scented
silhouette
soften/ softener
solemn
tragically
wharf
whichever
whip/ whiplash
whisk
whiten
whoever
wholly
wrench
wrestle
wrinkle/ wrinkled/ wrinkly
Advanced and Proficiency words with silent letters/ C1- and C2-level words with silent letters
abscess
acquit/ acquittal
aghast
aplomb
apostle
apropos
arraign/ arraignment
ascend/ ascent
awhile
bourgeois
bristle/ bristly
bustle/ bustling
cachet
catacomb
chez
crochet
debris
debtor
deign
depot
diaphragm
disciple
epistle
erstwhile
exhaust
faux
feign
fluorescent
ghastly/ ghastliness
ghoul/ ghoulish
glisten/ glistening
gnarl/ gnarly
gnash
gnat
gnaw
gnome/ gnomic
gnu
gristle/ grisly
hustle/ hustler
indict/ indictment
ironically
islet
isthmus
jamb
jeopardy
jostle
knack
knackered
knave
knead
knickknack
knoll
knuckle
laissez faire
limb
mnemonic
moisten
narwhal
nestle
numb/ numbness
parliamentary
pestle
phlegm
playwright
plumb
pneumatic
pneumonic
psalm
pseud
pseudo
psyche
psycho/ psychotic
rapport
redoubt/ redoubtable
rhapsodic/ rhapsody/ rhapsodise
rheumatoid arthritis
rhinestone
rhinoplasty
rhododendron
rhombic/ rhomboid
rhubarb
ricochet
rustle/ rustler
scenario
sciatic/ sciatica
scimitar
scintilla
scintillating
scion
shipwright
solemnity
soubriquet
subtle/ subtleness
succumb
thistle
tourniquet
trestle
valet
vehement
viscount
whack
whacko
wham/ whammy
wheedle/ wheedling
wheelie
wheeze/ wheeziness
whelk
wherein
whet/ whetstone
whey
whiff/ whiffy
whim
whimper
whimsy/ whimsical
whine/ whiny
whinge
whirl
whirr
whiskers
whist
whittle
whiz
whoop/ whoopee
whoops
whoosh
whopper/ whopping
whore
womb
wraith
wrangle/ wrangler
wrath/ wrathful
wreak
wreck/ wreckage
wren
wrest
wretch/ wretched
wriggle/ wriggly
wring/ wringer
writ
writhe
wronged/ wrongful
wrought
wrung
wry