Real future possibilities and their results
How English links a real future possibility to its result, and why one half of the sentence stays in the present.
If it rains tomorrow, we will stay at home.
The first conditional describes a real future possibility and its result. The condition clause, introduced by the conditional conjunction, stays in the present simple even though its meaning is future; the result clause takes the future auxiliary plus the base form of the verb. The two clauses can come in either order - when the condition comes first, a comma separates them; when it comes second, no comma is needed.
The strongest pull is to put the future auxiliary in both halves of the sentence, because many languages do exactly that. In English the condition clause never takes it - the present simple there already points to the future. Only the result clause carries the future marking.