I originally posted this as an answer to a forum question, however, i am leaving it as a tutorial for anyone that might be having a little difficulty with this subject. You can do it!
Both a and an are indefinite articles in English
As a general rule, a is used when the word or noun have a consonant sound ( meaning words that start with: s, t, w, any sound that is not a vowel) , an is used when the word or noun has a vowel sound ( meaning that they start with: a, e, i, o ,u ).
We use a like this:
She has aBachelor’s Degree.
Notice that all the words above start with a consonant.
We use an like this:
Notice that all the words above start with a vowel.
Remember, we choose a or an based on the word, and how we pronounce it. For instance, there are special cases like this:
Sometimes the H in words is silent, and this results in the word having a vowel sound , meaning that we need to use an.
anHour (Said like OUR, starts with a vowel sound)
anHeir (Said like EIR, vowel sound)
anHonest (Said like ONEST, vowel sound)
When H is not silent and has a consonant sound, we use a , like this:
aHouse (Said like JAUS, consonant sound)
aHome (Said like JOME, consonant sound)
There is another special instance. Some words beginning with the vowel U are enunciated with the sound /ju/ (like the word “you” or “Jupiter” ). In this case we use a since it starts with a consonant sound.
aUnicorn (Sounds like Younicorn, consonant sound)
aUnique (Sounds like YOUNIQUE, consonant sound)
It is perfectly normal to struggle to identify where to use a or an, don’t feel bad if you use them incorrectly. the more you practice, the more you will associate the correct use of the words, and you will be able to build more complex sentences with them.
I hope this helps, and if I missed anything or have further questions, I will try and answer as soon as possible