First of all: There are three types of conditional clauses: Realis of the present (type I), Irrealis of the present (type II) and Irrealis of the past (type III).
The good news first: The construction of conditional clauses in French is equal to the English one. As a big difference to other Roman languages, the subjonctif is not used in French conditional clauses, which makes our lives much easier. Realis
of the present is the condition in présent
de l' indicatif, the consequence is in présent de l' indicatif
(English: simple present / simple present or future; identical to French)
in Irrealis
of the past is the condition in imparfait
de l' indicatif, the consequence is in conditionnel
(English: subjunctive of the past = simple past / conditional I; identical to French)
in Irrealis der Vergangenheit
the condition is in plus-que-parfait de l'
indicatif, the consequence is in conditionnel passé.
(English: subjunctive perfect = past perfect / conditional II; identical to French)
Examples
Realis of the present (type I)
If he has time, he comes.
S' il a le temps,
il vient.
If we have time, we come.
Si nous avons le
temps, nous venons.
If I have the money, I buy a car.
Si j' ai de l' argent,
je m' achète une voiture.
If you have the money, you buy a car.
For this type I also the future can be used:
Examples
Realis of the present (type I)
If you need information, I am here.
Si tu as
besoin de quelques informations, je
suis
là.
If you need someone, I will be there.
Si tu as
besoin de quelqu' un, je serai
là.
In English as well as in French there is a difference between the condition (if = si) and the chronological narration of events (when = quand)
Examples
Quand
tu auras lu la lettre, tu seras d' accord
avec moi.
When you will have read the letter, you will agree with me.
The Irrealis der Gegenwart describes the possibility of the condition as possible but as little likely. This again means that the consequence is of little probability.
Examples
Irrealis of the present
S' il avait
le temps, il viendrait.
If he had time, he would come.
Si nous avions
le temps, nous viendrions.
If we had time, we would come.
Si j' avais
de l' argent, je m' achèterais
une voiture.
If I had money, I would buy a car.
Si vous aviez
de l' argent, vous vous
achèteriez une voiture.
If you had money, you would buy a car.
The Irrealis der Vergangenheit describes the possibility of the condition as non-existent, because it had been in the past. This also means that the consequence is impossible.
Examples
Irrealis of the past (type III)
S' il avait
eu le temps, il serait venu.
If he had had time, he would have come.
Si nous avions
eu le temps, nous serions venus.
If we had had time, we would have come.
Si j' avais
eu de l' argent, j' aurais
acheté une voiture.
If I had had money, I would have bought a car.
Si vous aviez
de l' argent, vous auriez
acheter une voiture.
If you had had money, you would have bought a car.