Why is the initial letter of some of the words like pneumonia, and psychology not pronounced?
It is an English phonetic adaptation of Greek words:
There are a number of Greek onset clusters imported into English:
'gn-' as in 'gnostic'
'pn-' as in 'pneumonia'
'pt-' as in 'pterodactyl'
'ps-' as in 'psychology'
- The 'g' and 'p' are not silent in Greek. But /gn/, /pn/, /pt/, /ps/ are not legal onsets in English phonology. English speakers naturally dropped the first sound to make it more compatible with English.