Some authors distinguish three (e.g. Thomson and Martinet 1960-69:§216), four (e.g. Nehls 1978:§3.6) or more types of conditional sentences, according to various syntactic or semantic criteria, but the simplest categorization is real vs. unreal (or "open" vs. "hypothetical--cf. Quirk et al. 1962:§11.31) conditions. In real conditions the speaker's attitude is neutral concerning the likelihood of the condition (expressed in the if clause) being or becoming true:
If John comes to the party, I'll leave.
Here the speaker makes a prediction about what he will do if John comes to the party, but expresses no opinion about the likelihood of John's coming. In unreal conditions the speaker expresses his doubt that the condition is or will be fulfilled:
If John came to the party, I would leave.
Here the speaker again predicts what he will do if John comes to the party, but also implies that he thinks it unlikely that John will come. 'If the time reference is past, the condition is still unreal, but since the outcome in this case is already known at the time of speaking, the speaker's attitude is better described as certain rather than doubtful about the fulfillment of the condition:
If John had come to the party (i.e. but he didn't), I would have left.