Real conditional
Real conditional sentences describe a reality-based situation. A condition that we think has been or could be fulfilled. We use the present tense in the main clause as well as the conditional clause.

Example:
Wenn ich im Lotto gewinne, mache ich eine Weltreise. If I win the lottery, I'll take a trip around the world.

Non-real present conditional
Unreal conditional sentences describe a condition not fulfilled in the present tense. In both sections of the sentence, we use the subjunctive II for situations in the present (or the would form).

Example: Wenn ich Geld hätte, würde ich eine Weltreise machen. If I had money, I would take a trip around the world.

Non-real past condition
Here we are describing a condition that was not met in the past. In both parts of the sentence, we use the subjunctive II for situations in the past.

Example: Wenn ich Geld gehabt hätte, hätte ich eine Weltreise gemacht. If I had had money, I would have taken a trip around the world.

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Notes on the construction
The conditional is in the subordinate clause and begins with wenn. Wenn is a subjunction so the finite verb must be at the end of the sentence. 

Example:
Wenn ich Geld habe, ... If I have money, ...
Wenn ich Geld  gehabt hätte, ... If I had had money, ...

The action that is dependent on the condition is in the main clause. When a sentence begins with a condition (dependent clause), the subject of the main clause is placed after the finite verb.

Example:

Ich mache eine Weltreise, wenn ich im Lotto gewinne./Wenn ich im Lotto gewinne, mache ich eine Weltreise.
I will take a trip around the world if I win the lottery./If I win the lottery, I will take a trip around the world.