Use a before consonant sounds and an before vowel sounds. Notice that some words whose first letter is a consonant (for instance, hour) actually begin with a vowel sound, and other words whose first letter is a vowel (for instance, unit) actually begin with a consonant sound.
a electron, a oxidant, a
an electron, an oxidant, an
an chromatograph, an human gene,
an unique solution
a chromatograph, a human gene,
a unique solution
Make your decision between a and an according to the first sound of the word that immediately follows a or an, regardless of the word's function.
a electric circuit, an very powerful
an electric circuit, a very powerful