Improve the bracketed part of the sentence by choosing the most appropriate option: “I could not help but (had to cry) at his sad story.” Select the alternative that makes the sentence grammatically correct and natural.

Introduction / Context:
This grammar question tests your understanding of the fixed expression “could not help but” and the form of the verb that follows it. Such idiomatic structures are an important part of English usage and are frequently examined in error-spotting and sentence-improvement questions. The sentence describes an emotional reaction to a sad story, and you must choose the form that fits the pattern correctly.

Given Data / Assumptions:

• The sentence is “I could not help but (had to cry) at his sad story.”
• The options for the bracketed part are “cry”, “cried”, “was crying”, and “no improvement”.
• The auxiliary verb “could” is already present before “not help but”, so the main verb that follows should be in its base form.

Concept / Approach:
There are two common patterns using “could not help” in English: “could not help doing something” and “could not help but do something”. In the second pattern, the verb that comes after “but” is always in the base form (bare infinitive) without “to”. The phrase “had to cry” introduces an unnecessary past tense and an extra auxiliary, which breaks the correct idiomatic structure. Therefore, the grammatically correct and idiomatic form here is “could not help but cry”.

Step-by-Step Solution:
Step 1: Recognise the idiom “could not help but + base verb”. It expresses that a person was unable to avoid doing something. Step 2: After “could not help but”, the verb should appear in base form. For example, “could not help but laugh” or “could not help but smile”. Step 3: The current phrase “had to cry” is in past tense with an additional auxiliary “had”, which does not fit the structure following “could not help but”. Step 4: Check each option: “cry” (base form), “cried” (simple past), “was crying” (past continuous), and “no improvement” (keeping “had to cry”). Step 5: Only “cry” fits the required base form after “could not help but”. Neither “cried” nor “was crying” matches the set idiom. Step 6: Replace the bracketed part with “cry” to form “I could not help but cry at his sad story”, which is both grammatically correct and idiomatic.

Verification / Alternative check:
Compare the revised sentence with similar examples from standard usage: “She could not help but laugh at the joke” and “They could not help but agree with the proposal.” In each case, “laugh” and “agree” are in the base form. By analogy, “cry” must also be in base form in this structure. The corrected sentence reads smoothly and is commonly used in both spoken and written English.

Why Other Options Are Wrong:

• cried: Simple past tense form which is not used after “could not help but” in this idiomatic expression.
• was crying: Past continuous tense; again, this breaks the fixed pattern and sounds unnatural.
• no improvement: Keeping “had to cry” would produce “I could not help but had to cry”, which is clearly incorrect because of the double auxiliary “could” and “had”.

Common Pitfalls:
Some learners mix the two patterns “could not help doing something” and “could not help but do something”. They attempt to combine them and produce incorrect forms. Remember: after “could not help but”, the verb stands in the bare infinitive. After “could not help”, when “but” is absent, a gerund is used, as in “could not help crying at his sad story”. Keeping these patterns distinct will help you avoid such errors.

Final Answer:
The correct improvement is to use the base form, giving “could not help but cry at his sad story.”