A conditional clause is an if clause. As in English, they can be positive and negative, they have two halves, and there is a word for if or if not present. In Latin those words are si and nisi.
Simply put, there are six types of conditional. The first three of which are very straightforward. You can have if clauses with the past, the present, and the future indicative: “If you go, I go” etc. It’s important to note that sometimes you’ll get only one half of the if clause, and that you can mix and match tenses as in English. “If you were upset, I’m sorry.” “If you want to go, fine.” If you spot a si or a nisi and the accompanying verbs are in the indicative, then your translation will come naturally once you pay attention to the tenses.