English shows how likely a condition is through the tense: if it rains (it may well) or if it rained (it isn't, or probably won't). German does the same through the mood of the verb:

Real — it could happen	Unreal — it isn't so
Wenn es morgen regnet, bleiben wir zu Hause.	Wenn es nicht regnen würde, würden wir an den See fahren.
If it rains tomorrow, we'll stay at home.	If it weren't raining, we'd go to the lake.
Wenn ich Zeit habe, komme ich vorbei.	Wenn ich Zeit hätte, würde ich vorbeikommen.
If I have time, I'll drop by.	If I had time, I'd drop by.

• Real conditions use the ordinary indicative in both halves — and German happily uses the present for the future: bleiben wir, komme ich.
• Unreal conditions use the Konjunktiv II in both halves.
• The word order is the same either way: verb last after wenn, then the main clause opens with its verb.

vorbeikommen — to drop by.