λέξεις – Ancient Greek vocabulary – Conjunctions, Conditionals ∞

Conjunctions connect words, phrases or clauses together, and, in Ancient Greek, function much as they do in English. Ancient Greek, however, uses them far more often, and frequently in subtler ways, than in formal written English. This is in part because Greek has a strong tendency to link clauses and sentences together. Whereas consecutive sentences in English are most often separated by a pause (period), many consecutive sentences in Greek are actually marked or offset by coordinating conjunctions, such as “and,” “but,” and “therefore”

Conditional sentences are "If ..., then ..." statements. They make a statement that if something happens, then something else will happen.
The 'if' clause is referred to as the 'protasis' by grammarians. It comes from the Greek words 'pro' (meaning before) and 'stasis' (meaning 'stand'). So the 'protasis' means 'what stands before' or 'comes first' as far as these two clauses are concerned. The 'then' clause is termed the 'apodosis'; it is what 'comes after' the protasis.

There are a number of different relationships that can exist between the protasis and apodosis. There can be some overlap between these three relationships.
a) Cause-Effect relationship, where the action in the protasis will cause the effect in the apodosis. Romans 8:13b, "...but if by the spirit you put to death the practices of the body, you will live."
b) Evidence-Inference type relationship, where the apodosis is inferred to be true based upon the evidence presented in the protasis. This will often be semantically the converse of the ‘Cause-Effect’ relationship. For example 1 Cor. 15:44, "If there is a soulish body, there is also a spiritual one."
c) Equivalence between the protasis and apodosis, which is actually a subset of the Evidence-Inference relationship. For example Gal. 2:18, "...if I build up again those things which I destroyed, I prove myself a transgressor."

First Class Condition - Is considered the 'Simple Condition' and assumes that the premise (protasis) is true for the sake of argument. The protasis is formed with the helping word ei ('if') with the main verb in the indicative mood, in any tense; with any mood and tense in the apodosis.
Second Class Condition - Is known as the 'Contrary-to-Fact Condition' and assumes the premise as false for the sake of argument. The protasis is again formed with the helping word ei ('if') and the main verb in the indicative mood. The tense of the verb (in the protasis) must also be in a past-time tense (aorist or imperfect). The apodosis will usually have the particle an as a marking word, showing some contingency.
Third Class Condition - Traditionally known as the 'More Probable Future Condition', the third class condition should actually be split into two different categories, the 'Future More Probable Condition' (indicating either a probable future action or a hypothetical situation) and the 'Present General Condition' (indicating a generic situation or universal truth at the present time). It is formed in the protasis using the word ean (ei plus an = 'if') and a verb in the subjunctive mood. The main verb of the protasis can be in any tense, but if the condition is a 'Present General', the verb must be in the present tense.
Fourth Class Condition - Is usually called the 'Less Probable Future Condition' and does not have a complete example in the New Testament. The fulfillment of this condition was considered even more remote than the Third Class Condition. It was formed with the helping word ei and the optative mood in the protasis. The apodosis had the helping word an and its verb was also in the optative mood.