Actually, it is very similar to English. We have three conditional sentences.

if + simple present for things that may happen.

If I see Tim, I will explain him your mistake.

The same example in French:

Si je vois Tim, je lui expliquerai ton erreur.

if + preterit modal for things that might happen.

If I won a billion euros, I would go to space.

And in French:

Si je gagnais un milliard d’euros, j’irais dans l’espace.

if + past perfect modal for things that may have happened but did not.

If I had been a king, I would have bought a golden sword.

The same in French:

Si j’avais été roi, j’aurais acheté une épée en or.

In this case, the conditional would be used if the condition was a false hypothetical. It isn’t used in a hypothetical whose truth is yet to be determined.

Si j’étais libre, je viendrais. Mais j’ai un autre rendez-vous, donc je ne viendrai pas.
If I was available, I’d come. But I have another meeting, so I won’t come.

(In this case, « j’étais libre » is the false hypothetical as described in the first sentence above. Why? The second sentence vindicates that you’re at a meeting so you were not in fact free.)

Si je suis libre, je viendrai. Je saurai demain si je suis libre ou non.
If I’m available, I’ll come. I’ll know tomorrow whether I’m available.

(Pay attention to viendrai: future indicative vs. viendrais: present conditional. There is no future conditional in French.)

In French, it is impossible to use the future or the conditional after si. Therefore, even if the condition applies to the future, we use the present in the condition clause. The same happens in English, by the way.

Such conditional sentences tend not to be in the present tense, because if the action is happening now, then you know whether the condition is true or not. So “Si je suis libre, je viens” can sound a little strange out of context. However there are situations where it can happen and is idiomatic:

• If the action is habitual, rather than punctual.

  L’association se réunit toutes les semaines. Si je suis libre, je viens.
  The organization meets every week. If I’m avaiable, I come to the meeting.

  It is also possible in this sentence to use quand, because the meaning ends up being the same: when a meeting is held and I’m available, I go. When quand is possible, we tend to use it in preference to si in such a situation. However there are cases where only si works.

  L’association se réunit deux fois par semaine. Si je suis libre, je viens le jeudi.
  The organization meets twice a week. If I’m available, I go to the Thursday meetings.

• In casual French, it is common to use the present instead of the future.

  Je viens demain.
  I’m coming tomorrow.

  Si je suis libre demain, je viens.
  If I’m free tomorrow, I’ll come.

• The hypothetical might be unknown because it concerns an action that is happening in a remote location. This obviously can’t happen in the first person.

  S’il est au courant du problème, je suis sûr qu’il y travaille.
  If he’s aware of the problem, I’m sure he’s working on it.