So far you have learned all the six vowels that are used in Persian. The three short vowel signs cannot come at the beginning of words. They are merely markers that have to be placed over (or under) a consonant to indicate the vowel pronounced after that consonant. Thus there is a special way to write them when they come in the beginning of a word.
The three long vowels (“ی”, “و” and “ا”) change form when coming at the beginning of a word too. The following table indicates how each of the six Persian vowels must be written when they come at the beginning of a word.
Forgetting about its original usage (a long vowel sounding like “ā”), you can consider the letter “ا” in all these cases (maybe not the fourth case) as a consonant that sounds like a glottal stop. Thus, a helpful interpretation of what happens is that since a word cannot literally start with a vowel, the glottal stop “ا” which is a consonant is written in the beginning of the word before the vowel. If all this sounds too complicated, simply ignore it and try to memorize the table.
Knowing that the first letter of a word is always a consonant, you can be sure that whenever the letter “و” comes as the first letter of a word, it is definitely going to be pronounced as “v” and not “u”. The same is true for “ی”; one can be sure that it is pronounced as “y” and not “i” when coming in the beginning of a word.