There are a few exceptions to the general rule of using a before words that start with consonants and an before words that begin with vowels. The first letter of the word honor, for example, is a consonant, but it’s unpronounced. In spite of its spelling, the word honor begins with a vowel sound. Therefore, we use an. Consider the example sentence below for an illustration of this concept.

My mother is a honest woman.
My mother is an honest woman.

Similarly, when the first letter of a word is a vowel but is pronounced with a consonant sound, use a, as in the sample sentence below:

She is an United States senator.