The subjunctive tenses of Portuguese are considerably different from those of English. The uses are also quite different, so it's doubtful that you'll find very much in English grammar that will help you with the Portuguese subjunctives. In English the present subjunctive of be is be , the past subjunctive is were .
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jcarp179past subjunctive as "that I owned,"Yes. That's just what I already told you above (minus that, which is the most usual complementizer with subjunctive clauses, and thus assumed).
jcarp179future subjective as "if I were to own."This is not a true subjunctive of "own". It's the subjunctive of be