Conditional (if) sentences.

• If I study, I will pass the exam. → Si estudio, aprobaré el examen. 
• If I studied, I would pass the exam. → Si estudiara, aprobaría el examen. 
• If I had studied, I would have passed the exam. → Si hubiera estudiado, habría pasado el examen.

There are 3 types of conditional sentences. Hay 3 tipos de oraciones condicionales:

1. condition is possible to fulfill - la condición es posible de cumplir 
• if-clause = Simple Present (Presente simple) 
• If I study,
• main clause = will-future or (Modal + infinitive)
• I will pass the exam 
2. condition is in theory possible to fulfill - la condición es en teoría posible de cumplir 
• if-clause = Simple Past (pasado simple)
• If I studied,
• main clause = would + infinitive *
• I would pass the exam
3. condition is not possible to fulfill (too late)  - la condición no es posible de cumplir (es demasiado tarde)
• if-clause = Past Perfect
• If I had studied,
• main clause = would + have + past participle *
• I would have passed the exam

* Se puede sustituir could o might por would y a veces es posible cambiarlo por should, may o must.

• I would pass the exam.
• I could pass the exam.
• I might pass the exam.
• I may pass the exam.
• I should pass the exam.
• I must pass the exam.

Además, la cláusula con "if" puede estar al principio o al final de la oración: 

• Cláusula al principio: (la oración lleva coma)
• If I study, I will pass the exam. 
• If I studied, I would pass the exam.
• If I had studied, I would have passed the exam. 
• Cláusula al final: (la oración no lleva coma)
• I will pass the exam if I study. 
• I would pass the exam if I studied. 
• I would have passed the exam if I had studied.