Topic: Right Form of Verb,

Best Answer Explained in Below-:

The phrase "He ran fast lest he — miss the train" employs the word "lest," which indicates a purpose or intention aimed at preventing something undesirable from happening. In this context, the intention is to avoid missing the train. The options presented—can, could, should, and would—each have different implications in terms of modality and time.

1. **Can** implies ability, which isn't the right fit here because it doesn't convey the sense of preventing failure.
2. **Could** suggests a hypothetical or potential situation that might not be directly relevant to his immediate action of running.
3. **Would** denotes a future consequence or condition, but it lacks the sense of urgency tied to the preventive nature of "lest."

4. **Should**, in this context, implies an obligation or a necessity, aligning closely with the intention expressed by "lest." It suggests an expectation that he ought to do something to prevent the undesired outcome of missing the train.

Thus, "He ran fast lest he should miss the train" effectively conveys that his swift action was motivated by the necessity to avoid the unfortunate event of missing his train.