It has to do with a Phonetics rule. The English semi-vowel "U" sounds as /ju:/ in the alphabet according to IPA . The "j" sound is a semi-vowel sound and the rule says that we must use the indefinite article A + consonant /semi-consonant /semi-vowel sounds e.g. A unique / university but AN + vowels. e.g. AN unbelievable story( here the pronunciation of "U" is different It is a vowel sound)

It's actually to do with how you pronounce the word. Unique is pronounced as "Yunique", with a "Yu" sound compared with "umbrella" which is pronounced with a short "u" sound. So you would say "an umbrella", "an ugly dog", "an uncle", "an unusual practice". whereas you would say "a useful tool", "a unique situation", "a utility vehicle". Hope that helps.

It's the same when you say "a universal principle" or "a university". But you would say "an utter success" or "an understanding person". It's 'a' or 'an' depending on sound. Universal/university/unique have a YOU sound. Utter/understanding have a UH sound. Perhaps this will make it more clear - in speaking, in the UK you will find people can say either 'a hospital' or 'an hospital'. The former has clear pronunciation, the latter sounds more like "an 'ospital". It's 'a' or 'an' in this case for the same reason - sound.

It is a bit complicated. The rule is that you use a before words that start with a consonant sound and an before words that start with a vowel sound Take some time to read the following http://www.quickanddirtytips.com/education/grammar/a-versus-an?page=all

Something that is unique is only one of a kind - 1.each person's signature is unique ( adjective ) 2.Because of the extreme cold, the Antartic is a uniquely fragile environment.(adverb ) 3.Each time I returned , I was struck by the uniqueness of Brazil and its people.( unount noun + of ) 4.you can use unique to describe things that you admire because they are very unusual and special eg.- John's vocals are just unique or Please see Collins English dictionary for more examples. best wishes Ciaran