A, an, one (grammar)

Remember:

• 'a' + consonant: a spider, a piece of chocolate, a lemon...

• 'sp', 'sm', 'sc'... etc are consonants, not 'esp', 'esm', 'esc'... : a spade, a smile, a scary thought...
• 'h' is a consonant: a helicopter, a hyena, a hamster... [except 'hour': an hour]
• 'u' is a consonant when pronounced 'yu': a university, a unique idea, a useful programme..

• 'an' + vowel: an animal, an idea, an experiment...
• 'u' is a vowel when pronounced 'a': an umbrella, an ugly person...

• 'one' is a number. It means: 'one, but not two or three'. The emphasis is on the quantity.
• I was sad because my hens only laid one egg. I was expecting more.