“Some words in English have silent letters. How can we know which letters in which words are silent?”
Unfortunately, the best answer to this question is “Become a professional etymologist!” Etymology is the study of the origins of words, and the truth is that this is more a question of etymology than of grammar.
To give an idea of how big an area we are considering here, according to Kent Jones, Education Committee, Esperanto Society of Chicago, “More than 60% of (English) words have silent letters.”
James Chandler observes “Many people are perhaps not aware of the astonishing fact that nearly every letter of the English alphabet is silent in some word.”
Here are three reasons why English has so many silent letters:
Old English was 90% phonemic (words sound the same as they look). But from the beginning of the 15th century, we began to borrow words from other languages. Because grammar and usage rules are different in other languages, adopted words did not follow the rules of English pronunciation.
The English language ‘borrowed’ the Latin alphabet, and so we have only got 26 letters to represent around 41 different significant sounds. This means that we must attempt to use combinations of letters to represent sounds.
In the Middle English Period William Caxton brought the printing press to England. As time passed, pronunciation continued to change, but the printing press preserved the old spelling. That’s why today we have words that end in a silent ‘e’, or have other silent letters in the middle, like ‘might’. In fact, modern day English is only 40% phonemic.
So are there any rules and can they help us? Axel Wijk (Regularized English, 1959, Stockholm: Almqvist & Wiksells) came up with over 100 rules for English spelling. It is claimed that by using these rules, you can spell up to 85% of the words in English with 90% accuracy. But is this really helpful? Basically, no! It gets so complicated that a much easier approach is to memorize sight words.
So you can see that unfortunately there is no clear way to know about all the silent letters in English. But is it a hopeless case? Well, the best we can do is to offer the following list of some silent letters:
Mb at the end of a word (silent b), e.g. comb, lamb, climb.
Sc at the beginning of a word followed by ‘e’ or ‘i’, (silent c), e.g. scene, scent, science, scissors (except for the word ‘sceptic’ and its derivations!).
Kn (silent k), e.g. knife, knock, know.
Mn at the end of a word (silent n), e.g. damn, autumn, column
Ps at the beginning of a word (silent p), e.g. psalm, psychiatry, psychology
Ght (silent gh), e.g. night, ought, taught
Gn at the beginning of a word (silent g), e.g. gnome, gnaw, gnu
Bt (silent b), e.g. debt, doubtful, subtle (but not in some words, e.g. ‘obtain’, ‘unobtrusive’!)
The letter H is silent in the following situations:
At the end of word preceded by a vowel, e.g. cheetah, Sarah, messiah;
Between two vowels, e.g. annihilate, vehement, vehicle
After the letter ‘r’, e.g. rhyme, rhubarb, rhythm
After the letters ‘ex’, e.g. exhausting, exhibition, exhort.