There are two types of conditional sentences: real situations and hypothetical ones. In order to get these down, you’ll need to have a firm grip of the present indicative, the future tense, the conditional tense, and the imperfect subjunctive. If you learn this basic formula, then you should be set.
If present indicative, then future tense.
If imperfect subjunctive, then conditional tense (or imperfect subjunctive, -ra endings only)
Here are a few examples:
Si yo tuviera más tiempo, trabajaría (trabajara) más de voluntario. If I had more time, I would do more volunteer work. (The implication is that I don’t and probably won’t have more time)
Si yo tengo más tiempo, trabajaré más de voluntario. If I have more time, I will do more volunteer work. (The implication is that it is a real possibility that I will have more time at some point.)
Si estudiamos, sacaremos buenas notas. If we study, we will get good good grades.
Si estudiáramos, sacaríamos (sacáramos) buenas notas. If we studied, we would get good grades.
Keep in mind that the order of the clauses doesn’t matter. For example, you could rephrase the sentences above as follows:
Sacaremos buenas notas si estudiamos.
Sacaríamos (Sacáramos) buenas notas si estudiáramos.