Remember that A(AN) means "one" or "a single". You cannot use A(AN) with plural nouns.
I saw a bears in Yellowstone National Park. Not Correct
I saw bears in Yellowstone National Park. Correct
If there is an adjective or an adverb-adjective combination before the noun, A(AN) should agree with the first sound in the adjective or the adverb-adjective combination.
He is anexcellent teacher.
I saw areally beautiful eagle at the zoo.
Use A before words such as "European" or "university" which sound like they start with a consonant even if the first letter is a vowel. Also use A before letters and numbers which sound like they begin with a consonant, such as "U", "J", "1" or "9". Remember, it is the sound not the spelling which is important. For example, "1" is spelled O-N-E; however, it is pronounced "won" like it starts with a "W".
She has a euro. Sounds like "yu-ro".
That number is a "1". Sounds like "won".
Use AN before words such as "hour" which sound like they start with a vowel even if the first letter is a consonant. Also use AN before letters and numbers which sound like they begin with a vowel, such as "F" or "8". Remember, it is the sound not the spelling which is important. For example, "F" is pronounced "eff" like it starts with an "E".
I only have an hour for lunch. Sounds like "au-er".
Does his name begin with an "F"? Sounds like "eff".
Some words such as "herb" or "hospital" are more complicated because they are pronounced differently in different English accents. In most American accents, the "h" in "herb" is silent, so Americans usually say "an herb". In many British accents, the "h" in "herb" is pronounced, so many British say "a herb". In some British accents, the "h" in hospital is silent, so some British will say "an hospital" instead of "a hospital".
In English, some nouns are considered uncountable such as: information, air, advice, salt and fun. We do not use A(AN) with these uncountable nouns. (Learn more about countable and uncountable nouns.)