1.  Letter "A"

     The letter A can be silent, especially in words that end in -ally.

For example : Romantically, musically, logically

2. Letter "B"
     The letter ‘B' – it’s always silent when it follows the letter ‘M’. Example: Climb, crumb, lamb, etc.
     The letter ‘B' is often silent when it’s before the consonant ‘T’.  
example: subtle, debt, doubt

3. Letter "C"
     Now the letter 'C' is often silent after the letter 'S' as in a very common example. They are quite tricky.
Example: Muscle, scissors, fascinate, scene, scenario.

4. Letter "D"
     The letter 'D' is always silent before the consonant 'N'
Example: Wednesday, handsome, sandwich.
And a ‘D' can be very quiet (not quite silent) but very quiet in front of a ‘G’
Example : Edge, knowledge,  bridge, hedge.

5. Letter "E"
     The letter ‘e’ can often be silent at the end of a word.
_Example : Clue, bake, taste.
But that ‘E' on the end, although it might be silent, it can affect the pronunciation of the vowel sound before it.
_Example :  Hop > hope, sit > site, hid > hide, slid > slide
But if the ‘e’ is the final letter in the word but it's the only vowel sound then it needs to be pronounced.
_Example : He, she, we – it has to be pronounced.

6. Letter "G"
     The letter 'G' can be silent when previously there are vowels.
Example: Sign, champagne, design, foreign.

7. Letter "H"
     The letter ‘H' is often silent too, often when it's following a 'W'.
Example: What, when, why.
And sometimes 'H' is not pronounced at the start of a word like honest and hour.

8. Letter "I"
    The letter ‘I' is not usually silent, it's usually pronounced, except in the word business.

9. Letter "K"
     The letter ‘K' is not pronounced when it comes before an ‘N'.
Example: Knife, know, knee, knock, etc.

10. Letter "L"
      The letter 'L' is always silent when previously there is a vowel.
Example: Could, should, talk, walk, etc.

11. Letter "M"
      The letter ‘M' is usually pronounced and so you don't have to to worry about the letter ‘M'.

12. Letter "N"
       The letter 'N' is always silent when previously there was the letter 'M'.
Example: damn, autumn, hymn, etc.

13. Letter "P"
       The letter ‘P’ can also be silent in words.
Example: Receipt, psychology and psycho.
         ‘P' can also be silent or it changes when it's next to a ‘H’ and together ‘PH' and often mean an ‘F’ sound. 
Example: Paragraph and telephone.

14. Letter "R"
       The letter 'R' is not silent in American English but can also be in English and Australian English. For example, in the word butter, the 'R' at the end of the word is not pronounced.

15. Letter "S"
      The letter 'S' can be silent if previously met a vowel.
Example: Island, debris and aisle.

16. Letter "T"
      The letter 'T' can also be silent.
Example:  Listen, castle, ballet, soften, etc.

17. Letter "U"
      The letter 'U' can also be silent.
Example: Guitar, guilt, tongue, guard, etc. 

18. Letter "W"
       The letter ‘W’ is silent when it’s at the start of the word before the letter ‘R’.
Example: Wrong, wrist, write, wrote.
     'W' also not pronounced in these common words: Who, whoever, whole, two, sword, answer.