English pronunciation is not always completely obvious! Here are some of the letter combinations with different sounds in different words.
-ough as off in coffee: cough, trough
-ough as uff in ruffian: rough, tough,enough,
-ough as ow in cow: plough, bough, slough
-ough as oo in too: through
-ough as oh: though, dough
-ought as ort in port: thought, ought, bought
-ought as out: drought
-aught as ort in port: slaughter, daughter
-aught as aft: laughter
-eight as ite in bite: height, sleight
-eight as ate: eight, weight
-ea as ee in feed: bead, meat, treat, heat, seat, read (present tense)
-ea as e in bed: head, tread, bread, sweat, threat, spread, read (past tense), instead
-ow as oh: know(n), flow, grow(n), blow(n), low, tow, own,
-ow as ow: cow, now, brow, town, gown, brown, frown, down
Some letters are not pronounced, for example:
the b at the end of climb, thumb, crumb, tomb, comb
the k at the beginning of words starting kn-, such as knee, know, knit, knock
the g at the beginning of words starting gn, such as gnome, gnat, gnash (exception: gnu)
the l in words such as talk, walk, stalk