Let's focus on one of the smaller grammatical errors that come up and can easily be missed. This can make a massive difference in your writing; especially if you're sending your work to an editor, or a lecturer for marking.
This rule can be a frustrating one and may seem pointless to you. However, it's really simple, and once you know it, that's it. I think that's why once you do know it, it's obvious when someone else has made that mistake.
The Simple Rule:
Please note: this rule is based on sound.
'An' Is used in front of a word that begins with a vowel (A,E,I,O,U)
'A' is used in front of a word that begins with a consonant (Any other letter)
See a pattern here?
Now with adjectives...
A funny elephant
(Even though elephant begins with a vowel, funny begins with a consonant, you determine which one you use (a or an) with the first word that follows)
An amazing singer
A little boy
An interesting article
Just remember, a and an are referring to a singular noun (referring to one thing), therefore you wouldn't use an or a when talking about a plural.
You wouldn't say: 'There was a dogs'
You would say: 'There was two dogs'
And finally, this is where things become a little bit more complicated...
There are some exceptions to this rule
This rule is based off sound, more than spelling. If a word begins with a vowel but sounds like it begins with a consonant, then you would use 'A'. And vice versa.
Euro - begins with a vowel, but is pronounced 'yu-ro'. Therefore it would be:
University - begins with a vowel but sounds like it's pronounced with a Y. Therefore it would be:
Hour - begins with a consonant but sound like it begins with an 'a'. Therefore it would be:
Then, of course we must consider how we pronounce words differently in different accents too (which I know, makes things even more confusing).
And finally, there are some words that won't use 'an' or 'a'. These words are uncountable, for example air, love, water, fun.