Step 1: Understand the sentence
The sentence implies extreme tiredness, making it difficult to keep eyes open. We need an adverb that conveys “almost not” or “with great difficulty.”
Step 2: Analyze each option
- Hardly: An adverb meaning “almost not” or “scarcely.”
Sentence: “She was so tired that she could hardly keep her eyes open.”
This means she could barely manage to keep her eyes open, which fits perfectly.
- Hard: An adverb meaning “with effort” or an adjective.
Sentence: “She was so tired that she could hard keep her eyes open.”
Grammatically incorrect as “hard” doesn’t modify “could” appropriately here.
- Barely: An adverb meaning “only just” or “almost not.”
Sentence: “She was so tired that she could barely keep her eyes open.”
This is grammatically correct and similar to “hardly.” However, “hardly” is more common in this context
- Clearly: An adverb meaning “in a clear manner.”
Sentence: “She was so tired that she could clearly keep her eyes open.”
This implies she could easily keep her eyes open, which contradicts the meaning.
Step 3: Conclusion
“Hardly” and “barely” both fit