Real conditions
Type 1 conditionals (condicionales reales) describe situations that may genuinely happen. The si clause (if) names the condition, the main clause names the result. This is the English first conditional: If it rains, I won't go out.
English if and when both take the plain present, but Spanish splits them. Si states a condition. Cuando (when) names a time, and for the future it needs the subjuntivo.
What follows in the main clause
Futuro: prediction
For a prediction about the future the main clause takes the Futuro simple.
Si hace sol, iremos a la playa.
If it's sunny, we'll go to the beach.
Si no te das prisa, llegarás tarde.
If you don't hurry, you'll be late.
Presente: habit, plan
For habits, general rules and plans the main clause is in the present too.
Si comes mucho, engordas.
If you eat a lot, you put on weight.
Si tengo tiempo, voy al cine.
If I have time, I go to the cinema.
Si mañana llueve, me quedo en casa.
If it rains tomorrow, I'm staying at home.
Imperativo: instruction
For a request or instruction the main clause takes the Imperativo.
Si vienes, trae comida.
If you come, bring food.
Si tienes hambre, come algo.
If you're hungry, eat something.
Word order
The two halves can swap places without any change in meaning. If the si clause comes first, a comma follows; if it comes second, there is none, as in English.
Si llegas tarde, te esperamos.
If you're late, we'll wait for you.
Te esperamos si llegas tarde.
We'll wait for you if you're late.
