'Si' means 'if' in French. Like in English, it is used to talk about things that could happen. This is a 'si' clause in English: 'If I was rich, I would buy loads of books.' To say this in French, we would use the construction Si + imperfect, + conditional. So in French, the sentence above would be "Si j’étais riche, j’achèterais beaucoup de livres." The order can be swapped around, like in English: "I would buy loads of books if I was rich" would become "J'achèterais beaucoup de livres si j’étais riche." The 'si' clause is a fantastic way of showing off your knowledge of tenses in both the written and oral exams. Learn a couple of example 'si' clauses that you can surprise your examiner with if you get stuck!By the way, if you're struggling to remember which tenses to use, you'll notice that they are the same in English. In the sentence above, 'I was' is an imperfect (compare "when I was young" with "quand j'étais jeune") while "I would" is a conditional ("I would buy a car" with "j’achèterais une voiture").