IN SOME CASES WE ADD 'A' BEFORE THE WORDS WHICH ARE STARTING WITH VOWEL SOUND ......HOW IS IT SO?
The following explanation might clear your doubt on how or why a few words begin with 'a' and a few begin with 'an'.
'Umbrella' comes under vowel sound as the sound being used is U. Such words start with 'an'. Examples of such words are ‘umbrella’, ‘umpire’, etc.
This can be well explained in contrast with words which begin with the letter U and having a consonant sound. Such words begin with 'a'. Examples of the same are ‘utensil’, 'Europe', 'Universe', etc.