A conditional sentence is a hypothetical (GGBB p. 689). Greek has first, second, and third class conditions.
A first class condition uses ει in the protasis.
A second class condition also uses ει but has αν in the apodosis.
A third class condition uses εαν in the protasis.
The “if…” clause is the protasis [also called the "antecedent"]; the “then…” clause is the apodosis [or the "consequent"].