English has two forms of the indefinite article: a and an.
　　In modern usage, the form a is used in front of words that begin with a consonant sound; an is used in front of words that begin with a vowel sound.
　　The following uses of an are nonstandard in modern English:
　　OK, I admit it, I don’t see why the iPad would be an useful device.
　　Plot is an unique feature that indicatesthe address of the place.
　　Found an useful paper on grid generation
　　Fastest way to find an unique element outof given numbers
　　We are an uniform based school and the design of our uniforms has been a careful and consultative process withexecutive, staff, student and community.
　　It may be that writers who put an infront of unique or useful have mis-understood the rule; perhaps they think that an goes in front of any word that begins with u, regardless of how the u is pronounced.
　　Although the letter u usually represents a vowel sound, it does not always do so. Such words as umbrella, undertaker,and ugly do begin with a vowel sound, [uh]. These words should be preceded by an:
　　an umbrella
　　an undertaker
　　an ugly dog
　　Sometimes, u represents aconsonant sound that incorporates the y sound heard at the beginning of yellow:
　　unique [yoo-neek]
　　useful [yoos-ful]
　　usual [yoo-zyoo-l]
　　I’ve never heard any one pronounce the word an in front of one of these words, although I suppose that somewhere in the world someone may talk that way. For a speaker who pronounces the word unique as [uh-neek] or [oo-neek], there would be some justification for writing “anunique feature.”
　　What I think is that some speakers say “auseful paper” but go to write it and think it “looks funny”with a instead of an.
　　In the case of an beforea word that begins with u, let your ear guide you:
　　a useful device, but an unusual device
　　a unique feature, but an ultra-interesting feature
　　a useful paper, but an undervalued paper
　　a unique element, but an unknown element
　　a uniform-based school, but an unconventional school