An apple is a fruit. Luther King, Jr. was an idol to many people. Elizabeth II is the queen. I ate a peach and an apple. Therefore, an is used because it is acting as an article proceeding a word beginning with a vowel sound .
Ferns make good house plants.
Teachers do not receive high salaries. Leaves falling off trees in autumn make me sad. Many politicians are lawyers . His grandparents were farmers . The is used to indicate a noun that has already been identified and is therefore specific.