In the introductory clause of wish, what is the difference between had done and would/could have done for the past subjunctive?

The core distinction between using "had done" and "would/could have done" in the introductory clause of a wish about the past lies in the locus of agency and possibility. The standard past subjunctive construction for an unreal past wish is "I wish (that) + subject + past perfect ('had done')." This structure expresses a direct regret or desire for a specific past action or state to have been different, where the outcome was, in the speaker's view, a settled fact. For example, "I wish I had studied harder" directly references a concrete, completed past event (not studying) and the speaker's present regret about it. The verb in the subordinate clause is squarely in the realm of factual past alteration.

The alternative formulation, using "would have done" or "could have done," introduces a layer of modality concerning willingness, capability, or hypothetical future-in-the-past action that did not materialize. It is not a simple substitute for the past perfect. This construction is typically employed when the wish implicitly or explicitly involves another agent's volition or capacity. For instance, "I wish you would have told me earlier" conveys a regret focused on the other person's perceived unwillingness or failure to choose to act. Similarly, "I wish I could have attended" centers the regret on a lack of ability or opportunity rather than on the bare fact of non-attendance. The nuance shifts from "this fact did not happen" to "the willingness or possibility for it to happen was absent."

Mechanistically, the "would/could have" structure can sometimes blur the line between a pure past wish and a reported irritation or unfulfilled expectation that persisted over a period leading up to the past reference point. While "I wish he had helped" regrets the single fact of him not helping, "I wish he would have helped" can subtly imply a wish that he had been willing to help during a span of time when help was needed. It is crucial to note that this usage, while common in informal spoken English, is often considered non-standard or colloquial by prescriptive grammarians, who would insist on "I wish he had helped" for formal writing. The "could have" variant is more universally accepted as it clearly denotes ability, a concept not directly expressed by the past perfect alone.

Therefore, the choice between these forms is not arbitrary but serves specific communicative functions. Opt for "had done" to express a straightforward regret about an unchangeable past fact. Reserve "would have done" to emphasize a lament over someone's (often another's) lack of willingness or refusal, acknowledging this usage's informal character. Use "could have done" to frame the regret specifically around missed capability or foreclosed opportunity. This distinction preserves clarity, with the modal verbs adding a dimension of commentary on the conditions of possibility or volition that surrounded the past event, rather than merely the event itself.