Final answer:
The correct interrogative pronoun to complete the sentence about clouds flying higher is 'Who'. It's crucial to differentiate between subject personal pronouns and object personal pronouns for proper grammar. Pronouns have three cases: subjective, objective, and possessive, which determine their function in a sentence.
Step-by-step explanation:
To complete the sentence with an interrogative pronoun, we should use 'Who' because we need a pronoun that can ask a question about the subject performing the action of flying. Hence, the correct pronoun to use in the sentence 'Who knows whether clouds fly higher during the day or at night?' is 'Who'.
Pronouns are essential in constructing meaningful and concise sentences. They act as substitutes for nouns to avoid repetition and to make sentences sound more natural. Understanding the correct use of subject personal pronouns versus object personal pronouns is vital for proper grammar. Subject personal pronouns are used when the pronoun is doing the action in the sentence, while object personal pronouns are used when the pronoun is receiving the action.
Pronouns have different cases: subjective, objective, and possessive. Knowing when to use each case is important for grammar and clarity in writing. For example, in compound structures, subjective case pronouns are used for compound subjects, while objective case pronouns are used for compound objects.