When I use a and when an before a word?

Yes, it is a simple grammar rule, but if you speak and write english only at school, you forget it so fast. I speak from the indefinite article 'a' and 'an'.

Normally stands for a :) Word the indefinite article 'a'. But if the word begins with a vowel (a, e, i, o, u), then you use 'an'.

Example:

• a car
• a boy
• a girl

• an apple
• an idiot
• an egg

Ok thats clear, but no grammar without an exception. Important is the pronunciation too! For example:

• a unit (Speaking as a 'j' -> a)
  
• an uncle (Speaking as a 'a' -> an)
• a human
• an hour
  

In Principle easy, but there are so many words where I not sure, because of the pronunciation. I think the best way to learn this grammar rule is to read a lot of english texts an write from time to time here in this blog :)