"Sis Puella Magica!" translation

[JCarter426]

Is "should" necessarily imperative? If I were to say, "I should be so lucky!", I would consider that roughly equivalent to "if only I were so lucky!"

Wikipedia says: "The auxiliary should is used to make another compound form that might be regarded as a subjunctive, and, in any case, it is frequently used as an alternative to the simple present subjunctive."

It might just be an ambiguity in English, "should" being used for different moods.

I suppose that's true. Your examples sound optative too, though.

You can't infer what it should be based on a translation/another language's patterns.

It's more that he disagrees with the English translation.

Present subjunctive in Latin often carries meanings that in the present linguistic theory would generally be analysed as optatives or jussives (depends on the subject). 2nd person present subjunctive is primarily optative with the jussive (basically polite imperative) meanings mostly occurring in poetry. Thus, the subjunctive actually means precisely the optative you mentioned in this case.

Anyway, there is a whole chapter of Latin grammar concerning the use of subjunctive within main clauses. This particular case seems to be an exhortation to me, because of where the verb is placed, but it could also be something else.
(Utinam) puella magica sis = O I wish you were a magic girl (which you can become, since it's in present form)
sis puella magica = come on, be a magic girl
sis puella magica, at stulta es = even if we admit that you are a magic girl, you're still dumb
sis puella magica = let's assume that you are a magic girl.

That sounds about right. I'll have to defer to you two. The reference I had on subjunctives gave no optative or polite imperative examples, although it did include negative imperatives.

We both really should know better, but like I said, years.