im righting a paper and i'm not sure what one to use for each situation
Not a teacher
-Both are used before a singular noun (countable noun) but there are still some special situation: 'a tea' (tea is an uncountable noun) means 'a cup of tea', 'a coffee' means 'a cup of coffee', 'a sugar' means 'a spoon of water'...
-'an' is used before a word that starts with the sound of a, e, i, o or u. Notice that it is the sound that is important, not the spelling: an apple, an hour, an MP (Member of Parliament)
-'a' is used when the pronunciation of the noun begin with a consonant: a cat, a university, a uniform