In English, articles are words that define a noun as specific or unspecific. The articles "a" and "an" are used before nouns to indicate one item. "An" is used when the noun begins with a vowel sound, and "a" is used when the noun begins with a consonant sound.
Let's look at the sentences given:
I saw (a / an) octopus. An octopus begins with a vowel sound 'o,' so the correct article is 'an.'
She has (a / an) doll. A doll begins with a consonant sound 'd,' so the correct article is 'a.'
We need (a / an) egg for breakfast. An egg begins with a vowel sound 'e,' so the correct article is 'an.'
He found (a / an) book in the library. A book begins with a consonant sound 'b,' so the correct article is 'a.'
There is (a / an) insect on the table. An insect begins with a vowel sound 'i,' so the correct article is 'an.'
Understanding the rules of using articles correctly helps in ensuring that sentences are grammatically correct and flow smoothly.