The purpose of this study is to analyse different types of Spanish conditional sentences with si, and specially to deal with the sihubierismo, an expression which refers to untrue conditions in the past. It is so named because its protasis usually begins with si hubiera. For example, Si hubiera llovido, yo no habria ido. (`If it had rained, I would not have gone.`) The sihubierismo has a clear tone of sadness, disappointment, regret or frustration. All of these emotions naturally spring from our common life. And so almost all of the human languages may have their own conditional sentences like the sihubierismo. This investigation on Spanish conditional sentences with si is believed to contribute not only to our understanding of important syntactic structures in the Spanish language, but also to the solution of various communication problems. This study is composed of three chapters, an introduction and a conclusion. The first part of those chapters aims to do careful analyses of the structure of Spanish conditional sentences with si. And we will examine the sihubierismo by dividing the second chapter into four sections: the substitution between two forms of imperfect subjunctive, -ra and -se; the interpretation of the protasis devoid of the corresponding apodosis; the explanation of the clause where the apodosis is used alone; and the use of the phrase "de + infinitive" that has a conditional value. The third chapter provides detailed analyses of conditional sentences of the Latin, the Italian, and the French, and also we will interpret them in comparison with those of the Spanish.