Breakdown of ¡Quién hubiera tenido una fuerza sobrehumana ayer!
un
a, an, some
quién
who
tener
to have
haber
to have (auxiliary)
Questions & Answers about ¡Quién hubiera tenido una fuerza sobrehumana ayer!
Why do we use the past perfect subjunctive (hubiera tenido) here instead of the imperfect subjunctive (tuviera)?
Because the wish is about a past event that is now impossible to change, as shown by the word ayer. The structure ¡Quién! + past perfect subjunctive expresses regret over an unattainable past wish. If you were wishing for superhuman strength right now, you would use the imperfect subjunctive: ¡Quién tuviera una fuerza sobrehumana hoy!
If the translation says 'If only I had had', why is the verb conjugated in the third-person singular instead of the first person?
Could I use hubiese tenido in this sentence?
Yes, absolutely. The past subjunctive has two sets of endings (-ra and -se). ¡Quién hubiese tenido...! is perfectly correct and means the exact same thing, although the -ra form seen here is generally more common in everyday spoken Spanish.