The words in bold in the sentences are called articles (a/an, the). We use articles before nouns.
1. We use a/an for indefinite singular nouns.
- Put a before nouns which begin with consonant sounds. For example: a book, a crocodile, a flower, a mouse, a snake, a shirt, a unicorn.
- Put an before nouns which begin with vocal sounds (a, i, u, e, o). For example: an ant, an architect, an elephant, an envelope, an hour, an gaji.
2. We use the for definite nouns.