Translate the following: If it hadn't rained, she would have gone to the park.

S'il n'avait pas plu, elle serait allée au parc.
Just as in the English example, this 'Si' clause requires the pluperfect tense and the conditional perfect. 'Si' clauses are constructed around two essential parts: the condition clause, in this example 'if it hadn't rained', and the result clause, 'she would have gone to the park', which explains what would have happened had the conditions of the 'si clause' been met.
Remember that as we are using the verb 'aller', the auxiliary must be a conjugated form of 'être' (in this case, the conditional form 'serait'). Equally, as the subject of the result clause is female, the past participle 'allée' must agree with the subject in gender (belonging as it does to the class of verbs which take être as an auxiliary).