First of all, it must be "If I were you". Being someone else is clearly a hypothetical situation (you can not really be her) and therefore requires the subjunctive mood. As for the part of "when they come back", let me quote a website on verb tenses (saves me typing ): No Future in Time Clauses Like all future forms, the Simple Future cannot be used in clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc.
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