Step 1: Understanding the Question:
The question has two parts: first, to identify the main reason for Matilda Loisel's downfall in "The Necklace," and second, to suggest how this fate could have been prevented.
Step 2: Key Concepts - Vanity and Honesty:
Matilda's ruin is a direct consequence of her character flaws, particularly her vanity and pride. The avoidance of this ruin hinges on the value of honesty.
Step 3: Detailed Explanation:
The primary cause of Matilda's ruin was her vanity and dissatisfaction with her humble life. Her intense desire to appear wealthy and glamorous at the party led her to borrow a diamond necklace. When she lost it, her pride and fear of being shamed prevented her from confessing the truth to her friend, Madame Forestier. This dishonesty led her and her husband into a decade of crushing debt and hardship to replace the necklace.
She could have easily avoided this ruin by simply being honest. If she had told Madame Forestier immediately that she had lost the necklace, she would have discovered that it was a fake, worth very little. This simple act of honesty would have saved her from ten years of poverty and suffering.