In this English grammar sentence improvement question on conditionals, you must choose the correct verb form to express an unreal past condition. Original sentence: He would surely assist me if I had requested him to do so.

Introduction / Context:
This question tests your understanding of conditional sentences, especially the third conditional, which talks about unreal situations in the past. The speaker is imagining what would have happened if a different action had been taken earlier. Choosing the correct combination of "would have" and "had" forms is essential for accurate English grammar in such cases.

Given Data / Assumptions:
- Original sentence: He would surely assist me if I had requested him to do so.
- The condition refers to a past action that did not actually happen.
- The result also refers to a past possibility, not a present or future one.

Concept / Approach:
The third conditional structure in English is: If + past perfect (had + past participle), would have + past participle. It expresses a hypothetical result in the past that did not occur because the condition was not fulfilled. So the correct pattern is "He would have assisted me if I had requested him to do so." The use of "would assist" suggests a present or future result, which is not correct here.

Step-by-Step Solution:
Step 1: Identify the part of the sentence that shows the condition: "if I had requested him to do so". This is already correct as a past unreal condition. Step 2: Recognise that the result clause must match this unreal past situation. Step 3: Apply the third conditional pattern: If + had requested, then would have assisted. Step 4: Option B, "He would have surely assisted me if I had requested him to do so", follows exactly this pattern. Step 5: Check that the adverb "surely" does not disturb the structure and that the meaning is clear.

Verification / Alternative check:
We can compare with standard examples such as "If I had known, I would have helped you" or "If they had arrived earlier, they would have caught the train". In each case, both parts refer to unreal past events. Similarly, here we are speaking about a help that did not occur because the request was never made. Therefore, "would have assisted" is the correct verb choice, confirming Option B.

Why Other Options Are Wrong:
Option A, "would surely assisted", is grammatically wrong because "would" must be followed by the base form "assist", not "assisted". Option C, "will have surely assisted", mixes future perfect with a past unreal condition and sounds unnatural. Option D, "No improvement", keeps the incorrect structure "would surely assist" for a past unreal result. Option E completely changes the meaning to an actual event in the past, not a hypothetical one, so it does not match the intended conditional meaning.

Common Pitfalls:
Learners often confuse second and third conditionals, mixing "would" and "would have" forms. Another common error is placing the adverb between "would" and "have" and then dropping "have" accidentally. Always remember that for unreal past situations you need "would have" plus past participle in the result clause, together with "had" plus past participle in the if clause.

Final Answer:
The correct improved sentence is He would have surely assisted me if I had requested him to do so.