We had a really good science lesson at school today. (It's one lesson among many.)
I found an unusual insect on the wall outside our house. ("Unusual" starts with a vowel sound.)
It's your birthday next week. Are you going to have a party? (One party.)
We waited for hours, but we finally saw the Queen. (A specific, well-known person.)
Why don't we listen to the radio? (Refers to the radio as a device.)
Have you got a euro I could borrow? (One euro.)
Mum has gone to the bank, but she'll be back soon. (A specific bank, likely the one they usually use.)
Where have you been? I've been waiting for over an hour! ("Hour" starts with a vowel sound.)