There is often confusion over when to use ‘a’ or ‘an’.
Generaly we use ‘a’ with nouns starting with consonants and use ‘an’ with nouns starting with vowels. Consonants are b,c,d,f,g,h,i,j,k,l,m,n,p,q,r,s,t,v,w,x,y,z and vowels are a,e,i,o,u.
However, there are some other rules that you should be aware of.
Rule 1: Use a/an to describe what your job or occupation is.
For example: I am a teacher. She is a doctor. He is an accountant. I am an oncologist.
Rule 2: Use a/an to describe something general – NOT specific.
For example: Put the books on a table. (The books can be placed on any table= not specific) vs Put the books on the table (Both speakers know which table is being refered to= specific)
For example: There is a broken light in my class room. (We know that a light is broken but we do not know which one)
Rule 3: Use a/an to give a description of something.
For example: This is a computer. Tomatoes are a type of fruit. That is an apple. Articles are a type of determiner.
Rule 4: Use a/an when there is only one of something (singular)
For example: I need to eat a banana. There is a dictionary on the table which you can use. I have a pair of jeans, a T-shirt and socks.