Here's why:
* Subjunctive mood expresses a hypothetical or unreal situation. In this sentence, Sandy is not actually present, so the situation is hypothetical.
* Indicative mood describes a fact or a real situation.
* Imperative mood gives a command or request.
The subjunctive mood is often used in "if" clauses to express a condition that is contrary to fact. In this case, the sentence is saying that if Sandy were present, the situation would have been different.