Conditional tense is often used in sentences with subordinate clauses. (To a lesser degree, this is also true of future tense.) Subordinate clauses in the conditional are always paired off with the past-tense main clause. Future-tense subordinate clauses go with a present-tense main clause. Here is one common example:

Yo sabía lo que harían.

I knew what they would do.

Yo sé lo que harán.

I know what they will do.

If, Then

Future and conditional are also used in “if … then” clauses. When the “if” clause is in the present tense, the “then” clause has a future-tense verb:

Si consigo trabajo, compraré un coche nuevo.

If I get a job, I will buy a new car.

The conditional version of this is a little trickier — you'll need to use the imperfect subjunctive (presented in Appendix A) in the “if” clause:

Si fuera rico, compraría un coche nuevo.

If I were rich, I would buy a new car.