Understanding the Correct Answer: "didn't"
The sentence "I _____ use to make so many mistakes" requires a form of the verb "to do" in the past tense to convey a past habit or situation. The correct answer is option 'A': "didn't." Here’s why:
1. Past Tense Context
- The phrase "use to" indicates a habitual action in the past.
- To express this in the negative form (indicating that the action did not occur), we use "didn't."
2. Structure of the Sentence
- The correct formulation is: "I didn't use to make so many mistakes."
- "Didn't" is the contraction of "did not," which is used to negate the verb in the past tense.
3. Other Options Explained
- a) don't: This form is present tense and does not fit since the sentence refers to past habits.
- b) hadn't: This is the past perfect form, which is not appropriate in this context as it suggests that the action was completed before another past action.
- c) wouldn't: This implies a conditional sense and does not indicate a past habit.
4. Conclusion
- The use of "didn't" effectively communicates that in the past, the speaker made many mistakes, but in the context of the sentence, they did not have that habit anymore. This grammatical structure helps convey changes in behavior over time.
Understanding these distinctions is crucial for mastering English verb tenses and their appropriate usage in sentences.

To solve this question, we need to identify the correct verb form to fill in the blank. Let's analyze the given options:
A: didn't - This is the correct answer. The verb "didn't" is the contraction of "did not" and is used to indicate the past tense of the verb "do."
B: don't - This is incorrect. The verb "don't" is the contraction of "do not" and is used to indicate the present tense of the verb "do."
C: hadn't - This is incorrect. The verb "hadn't" is the contraction of "had not" and is used to indicate the past perfect tense of the verb "have."
D: wouldn't - This is incorrect. The verb "wouldn't" is the contraction of "would not" and is used to indicate the conditional or future tense of the verb "will."
Therefore, option A: "didn't" is the correct answer to fill in the blank. The complete sentence would be: "I didn't use to make so many mistakes." This indicates that in the past, the person used to make a lot of mistakes, but not anymore.