Theory

One of the most important elements to understand in the pronunciation of English words is the effect of silent letters.
Many words are pronounced as they are because of this.
For example:
In the word ANSWER, there is an obvious silent letter, the W.
But the R at the end is also silent.
The word really sounds like A-N-S-A

Every letter, from A to Z, except for V, is silent sometime.
For example:
A is silent in AISLE
B is silent in THUMB
C is silent in MUSCLE
D is silent in SANDWICH
E is silent in GATE
F is silent in CLIFF
G is silent in SIGN
H is silent in ANCHOR
I is silent in JUICE
J is silent in MARIJUANA
K is silent in KNIFE
L is silent in TALL
M is always silent before N, asin MNEMONIC
N is silent in COLUMN
O is silent in PEOPLE
P is silent in RECEIPT
Q is silent in LACQUER
R is always silent when followed by a consonant, as in PARK.
It is not silent when followed by a vowel, as in PRAY, but it is silent at the end of a word, like FOUR.
S is silent in ISLAND
T is silent in WHISTLE
U is silent in BISCUIT
V is never silent.
W is silent in SWORD
X is silent in FAUX PAS
Y is silent in MAYOR
Z is silent in BUZZ

Silent letters can also influence the sound of other letters in a word, and E is one of the main letters with this function.
For example:
HAT which sounds like CAT becomes HATE which sounds like PLATE because of the silent E.

Some letters can have more than one sound.
For example:
C in CINEMA or C in CAKE.
G in GERRY or G in GAME

In many such cases, silent letters are used to show how the letter should sound.
For example:
The U in BISCUIT is there to show that the C is as in CAKE and not cinema.
The U in GUARD is the to show that the G is as in GAME and not GERRY.

Sometimes a letter can be silent or not, both cases being acceptable.
For example:
The T in OFTEN can be silent or not, it doesn't matter.