Si clause in the present, followed by main clause in the present
Si ustedes están cansados, pueden descansar un poco aquí. If you are tired, you can rest a little here.
Si clause in the present, followed by the main clause in the future
Si Andrés sigue el sendero llegará al campamento en dos horas. If Andrés follows the path, he will arrive at the camp in two hours.
Si clause in the present, followed by Ir a + infinitive
Si no lleva usted un suéter en su mochila, va a tener frío. If you don’t carry a sweater in your backpack, you are going to be cold.
Si clause in the present, followed by a command
Si el cartón está vacio, ponlo en el basurero. If the carton is empty, put it in the wastebasket.
Si clause in the imperfect followed by the main clause in the imperfect*
Si María tenía el dinero, siempre iba al cine los domingos. If María had the money, she would go to the movies on Sunday.
*Note: When the si clause speaks of a habitual past action that is not contrary to fact, the imperfect tense is used in both clauses.