We've all done it one time or another. Instead of enunciating the syllables in "probably," a slurred "probly" comes out instead. Why does this happen? It's really a question of efficiency. English ...
This can make learning to spell the words quite tricky! Watch this short video to see some more examples of silent letter words. Vowel sounds are made by the letters 'a', 'e', 'i', 'o' and 'u'.