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 firstletter is a vowel (for instance, unit) actually begin with a consonantsound.
a electron, a oxidant, ahour
an electron, an oxidant, anhour
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 firstsound of the word that immediately follows a or an,regardless of the word's function.
a electric circuit, an very powerfulexplosive
an electric circuit, a very powerfulexplosive
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